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Facing Challenges in Real-Life Application of Surface-Enhanced Raman Scattering (SERS): Design and Nanofabrication of SERS Substrates for Rapid Field Test of Food Contaminants Ruyi Shi, Xiangjiang Liu, and Yibin Ying J. Agric. Food Chem., Just Accepted Manuscript • DOI: 10.1021/acs.jafc.7b03075 • Publication Date (Web): 18 Sep 2017 Downloaded from http://pubs.acs.org on September 18, 2017
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Journal of Agricultural and Food Chemistry
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Facing Challenges in Real-Life Application of
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Surface-Enhanced Raman Scattering (SERS):
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Design and Nanofabrication of SERS Substrates for
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Rapid Field Test of Food Contaminants
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Ruyi Shi,† Xiangjiang Liu,† Yibin Ying*,†, ‡
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†
College of Biosystems Engineering and Food Science, Zhejiang University, 866 Yuhangtang
7 8 9
Road, Hangzhou, Zhejiang 310058, China ‡
Zhejiang A&F University, 88 Huanchengdong Road, Hangzhou, Zhejiang 311300, China
* Corresponding author: Email:
[email protected] (Y. B. Ying)
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RECEIVED DATE (to be automatically inserted after your manuscript is accepted if
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required according to the journal that you are submitting your paper to)
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ABSTRACT
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Surface-enhanced Raman scattering (SERS) is capable of detecting single molecule with high
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specificity and has become a promising technique for rapid chemical analysis of agricultural
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products and foods. With a deeper understanding of the SERS effect and advances in
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nanofabrication technology, SERS is now on the edge of going out of the laboratory and
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becoming a sophisticated analytical tool to fulfill various real-world tasks. This review focuses
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on the challenges that SERS has met in this progress, such as how to obtain a reliable SERS
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signal, improve the sensitivity and specificity in a complex sample matrix, develop simple and
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user-friendly practical sensing approach, reduce the running cost, etc. This review highlights the
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new thoughts on design and nanofabrication of SERS-active substrates for solving these
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challenges and introduces the recent advances of SERS applications in this area. We hope that
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our discussion will encourage more researches to address these challenges and eventually help to
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bring SERS technology out of the laboratory.
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KEYWORDS: SERS, SERS-active substrate, nanofabrication, food contaminant, rapid field test
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Journal of Agricultural and Food Chemistry
1. Introduction
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Surface-enhanced Raman scattering (SERS) was first observed in 1974 by Fleischmann when
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pyridine was adsorbed on a roughened silver electrode1 and was correctly interpreted later by
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Van Duyne and Creighton in 1977.2,3 Since its discovery, SERS remained interesting only to a
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relatively small scientific community, e.g., to the electrochemistry or Raman spectroscopy fields.
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It was not until the discovery in 1997 that SERS can boost the weak Raman signal to a
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surprisingly high level, which is sufficient for single-molecule detection,4,5 that it suddenly
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attracted enormous attention. Currently, due to the high sensitivity and high specificity inherited
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from the unique molecular spectral fingerprints of the Raman spectrum, a large range of
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disciplines (including chemistry,6,7 physics,8 materialogy9 and life sciences10) have begun to
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exploit the huge potential of this technique.
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Specifically, in analysis of foods and agricultural products, SERS technique has been used as a
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rapid and sensitive tool for detecting chemical and microbial hazards. Compared with the well-
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established “wet chemistry” methods and modern analytical instrumental techniques (HPLC,
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GC-MS, etc.), SERS can avoid complicated and time-consuming sample pretreatment, therefore
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being more preferred in rapid field screening. Another advantage of SERS is that it has less
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interference from water, which is very important in analyzing biological samples such as
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agricultural products. Various food contaminants, such as pesticide,11–13 illegal food
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additives,14,15 veterinary drug,16,17
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technique. Besides, SERS has also been developed as a rapid diagnostic tool for food-borne
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pathogenic microorganisms.20,21 In all, SERS shows great promise in rapid safety assessment of
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foods and agricultural products.
toxins,18,19 have been successfully detected by SERS
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Several excellent review articles related to SERS have been recently published. Most reviews
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focus on the mechanism of the SERS effect,22–24 applications of SERS in various fields,25–33
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fabrication of SERS-active substrates,34–36 and advances in new SERS devices,37–41 which are of
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interest to a heterogeneous readership. Naturally, this review cannot cover the entire picture of
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the corresponding topics of SERS, but instead presents the reader with the recent advances of
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SERS from a different angle.
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With a deeper understanding of the SERS effect and advances in nanofabrication technique,
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SERS is now on the edge of going out of the laboratory and becoming a sophisticated analytical
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tool to fulfill various real-world tasks. This review focuses on the major challenges that SERS
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has met in this progress, such as how to obtain a reliable SERS signal, how to reduce the running
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cost of this technology, etc, which are the keys to developing any practical applications.
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Specifically, in analysis of food contaminants in these real-world samples (e.g. fruits, vegetables,
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meats, oils, beverages, etc.), how to increase specificity and avoid interferences from complex
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sample matrix, and how to simplify the sampling and practicing approach become especially
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important. This review highlights the new thoughts on design and nanofabrication of SERS-
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active substrates for solving these challenges and introduces the recent advances of SERS
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applications in the analysis of agricultural products and foods, as well as the future developments
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and several prospects of SERS.
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2. Brief Introduction of SERS
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It has been generally accepted that two enhancement mechanisms may contribute to the
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enormous enhancement of SERS: an electromagnetic (EM) mechanism42–44 and a chemical
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enhancement.45 The former one is considered as the dominant reason for SERS,46 which
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originates from collectively oscillating conduction electrons in metal or metal-like nanomaterials
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under light excitation (see Figure 1a, b), the so-called surface plasmon resonance (SPR),
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including localized surface plasmon resonance (LSPR) and propagating surface plasmons
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(PSP).47–50 This effect can focus the incident light to nanoscale edges, gaps, tips, or crevices,
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therefore creating many electromagnetic “hotspots” in these sites, whose intensity can achieve 2-
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5 orders of magnitude of the incident light (Figure 1c).51,52 Molecules trapped in these hotspots
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can experience an enormous enhancement of their Raman intensity.4 Therefore, it would be more
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appropriate to call this effect plasmon-enhanced Raman scattering (PERS).50 However, we still
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use the common term, SERS, in this review, considering its long history and broad acceptance.
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For interested readers, these fundamental aspects of SERS can be referred to the review by
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Schlücker.53
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These metallic plasmonic nanostructures, also referred to as SERS-active substrates, are the
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prerequisite for observing SERS. The performance of a SERS-active substrate is usually
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evaluated based on two aspects: the enhancement factor (EF) and signal uniformity. Generally,
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the EF of a SERS substrate determines how sensitively it can detect a certain analyte. The EF is
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approximated to be proportional to the fourth power of the enhancement of the local
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electromagnetic field (|E/E0|4), where E and E0 are the intensities of the local electromagnetic
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field in the presence and absence of the substrates. The experimental determination of EFs
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requires measurements of the SERS intensity for the adsorbed molecules on the substrate relative
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to the normal Raman intensity of the same molecule using the following equation,
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EF =
ூೄಶೃೄ /ேೄಶೃೄ ூೃೄ /ேೃೄ
,
(1)
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where ISERS is the total SERS intensity and IRS is the normal Raman intensity of the probe
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molecules. NSERS and NRS are the numbers of molecules contributing under SERS and normal
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Raman conditions, respectively. The EF in the hotspot can be as high as 1014, which is sufficient
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for single-molecule detection.4,5 However, in most cases, the average EF of the entire SERS
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substrate is approximately 105-108.43,46,54 To further increase the EF of a SERS substrate requires
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a rational design and precise fabrication. However, it is a formidable task since so many factors
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need to be considered, such as the incident laser wavelength, optical properties of the analyte, the
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material of the substrate and, most importantly, the morphology of the metal plasmonic
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nanostructures of the substrate.55
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The signal uniformity of a SERS substrate is another crucial parameter since it decides how
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reliable a SERS measurement can be, which is the key to quantitative analysis. However, the
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SERS approach is known for poor reproducibility, mainly aroused by the traditional colloidal
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SERS substrates. To improve the reproducibility, a variety of approaches have been employed,
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such as controllable assembly, introducing an internal standard and employing a highly ordered
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SERS substrate, which we will address later. Evaluation of the reproducibility of SERS-active
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substrates is usually carried out by the relative standard deviation (RSD) value of the SERS
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intensity obtained on randomly selected spots. However, the complexity of evaluation the EF and
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the signal uniformity of a substrate should not be underestimated since several factors need to be
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taken into consideration together,56,57 such as the excitation wavelength and LSPR modes, the
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choice of probe molecule (avoiding resonance Raman effect), determination of the coverage of
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the probe on the substrates, etc. A full consideration of these factors should be given before
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comparing different SERS substrates.
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SERS-active substrates are the foundation for any SERS applications. An ideal SERS-active
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substrate generally needs to possess ultrafine features, large-scale uniformity, great EF, good
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signal reproducibility and a low fabrication cost. Thus far, the development of SERS-active
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substrates has gone through four stages: (1) metallic colloid substrates; (2) highly reproducible
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rigid substrates; (3) mechanically flexible substrates; and (4) universal “all-task” substrates
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(Figure 2). We will later discuss how to use these substrates to deal with the challenges in
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developing real-life applications and highlight the recent breakthroughs aiming to bring the
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SERS technique out of the laboratory.
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3. Challenges in Real-Life SERS Applications
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Despite the long history since its discovery, SERS has not yet become a sophisticated tool for
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practical applications. A major obstacle has been the poor reproducibility and poor stability of
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the wildly used metallic colloid substrates, limiting its applications in quantitative analysis. Also,
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when applying SERS technology to real-world applications, the cost is another major concern.
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Additionally, for practical applications, sample matrix interference cannot be overlooked. Since
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target molecules that are usually in low concentration and the matrix has the same access to the
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hotspots, a significant spectral “contamination” can often be observed in a complex sample
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matrix, which causes difficulty in analyzing the results and decrease of the sensitivity and
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specificity. Needless to say, there are many more challenges need to be addressed before further
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expanding SERS to real-life applications.
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3.1 Strategies to Improve the Signal Reproducibility of SERS
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Although the discovery of SERS was made on electrochemically roughened silver electrodes,1
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the first type of wildly used SERS-active substrates were metallic nanoparticles, e.g., gold, silver,
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and copper nanoparticles, freely suspended in a homogeneous medium. Due to the advantages of
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easy preparation, low cost, and tunable optical properties, these substrates have been preferred
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for several decades and extensively used in studies. These substrates significantly reduced the
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technical barrier for performing a SERS experiment and made SERS popular. For the preparation
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methods of various metallic colloids, the reader can refer to previously published reviews.58,59
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However, the EFs from single isolated nanoparticle are relatively low, which can be further
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increased by two approaches: either by increasing the intensity or the density of the EM hotspot.
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Because of the “lightning rod effect”,51 metal nanoparticles with sharp edges and corners exhibit
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a much higher enhancement of the local EM field compared to the spheres.60 Therefore, various
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heteromorphic nanoparticles have been employed to improve the EF, such as a triangle,61
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polyhedron,62 rod,63,64 cube,62,65 wire,66 star,67,68 popcorn,69 dendrite70 and so on. Another
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approach is to increase the density of hotspot in the substrates, by adding salt (e.g., NaCl, KCl, or
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NaNO3) to trigger the aggregation of nanoparticles. This process generates abundant hotspots
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between the gaps of aggregated nanoparticles. Aggregation can also be induced by centrifugal
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force71 and magnetic force.72,73 Sometimes, aggregation is inevitable, as simply introducing the
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analyte can induce it.74 However, these hotspots generated by aggregation are randomly
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organized and impossible to control (Figure 3a),75,76 and the aggregated nanoparticles are not
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stable and tend to sediment under gravity,77 causing both a fluctuation of the strength and
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number of EM hotspots in the detecting volume. Thus, the obtained SERS signals vary
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significantly from experiment to experiment and from time to time. It is quite often that the
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signal variation exceeds more than 50% using aggregated NPs.75,78 Therefore, it is almost
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impossible to use them for quantitative analysis. This is the reason that SERS gained its fame for
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lacking reproducibility.
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Controllable Assembly. To solve this problem, one of the strategies by using a controllable
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assembly technique is to synthesize well-defined superstructures based on small nanoparticles.
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With the help of some ligands, such as organic molecules,79–81 polymers,82 antibody/antigens,83
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biotin/avidin connectors,84 nanoparticles can be assembled into ordered superstructures, such as
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dimers or trimers,85 core-satellites,86–89 etc. One smart approach is based on the unique base-
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pairing rules and structural features of DNA, which can be used to program the assembly of
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ordered plasmonic superstructures,90 such as dimers,91–95 trimers,96 regularly spaced
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nanoparticles chains97 and two- or three-dimensional ordered arrays98–104 (Figure 3b). Another
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interesting approach is the liquid/liquid interfacial assembly technique. Based on soft ligand-
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ligand interactions, including steric hindrance, hydrogen bonding interactions, electrostatic
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forces,105–107 a variety of morphologies and structures have been successfully assembled, such as
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nanosheets,108–112 nanoribbons,113 folding structure,114,115 spherical supercluster,116 liquid-like
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metal droplets,117–120 and so forth (Figure 3c). These densely packed interfacial nanoparticle
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films have also been developed as SERS-active substrates.106,107 Due to dynamic nanogaps in
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the liquid interfacial structure, interfacial GNP arrays can cause surrounding molecules to easily
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diffuse into the nanogaps (hotspot).121 In these superstructures, the hotspots are evenly
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distributed, which ensures the good reproducibility of the signal (RSD% below 10%).121,122 Thus,
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these interfacial nanoparticle films overcomes the main drawbacks of conventional aggregated
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colloid substrates. However, the ligands required in the assembly process usually are strongly
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adsorbed on the nanoparticle and rest in the hotspot, therefore preventing the analyte from
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entering the hotspots for detection. Removing the ligands often involves harsh treatments and
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can cause complete or partial disassembly of the superstructures, therefore, limiting their
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practical applications.
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Internal Standard. A smart strategy for reducing the fluctuation of SERS intensity caused by
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uncontrollable aggregation of nanoparticles is to use an internal standard. The internal standard
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and target molecules normally have similar molecular structures and coexist in the same physical
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and chemical environment of the hotspot; therefore, its signal can be used to calibrate the
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absolute signal of each SERS measurement.123 It has been proven that the implementation of an
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internal standard can reduce the SERS signal fluctuation to the 10% level,124 which is enough for
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quantitative analysis. However, the reproducibility of this approach could be compromised, due
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to an unstable internal signal caused by interference from the external environment or
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competitive adsorption with the analytes, etc. In recent years, several new types of internal
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standards have been introduced (Figure 4). A special core/shell nanoparticle with internal
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standards embedded was developed (Figure 4a). Since internal standards are embedded inside
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the shell, they will not compete with the target molecules for the surface hotspots and will not be
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influenced by the external environment, providing a stable signal for calibration.125–132 For
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example, after correction with internal standards, the reproducibility of SERS signals between
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batches was significantly improved with a RSD% less than 8% (Figure 4a).126 This approach has
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also been used to develop a real-life application, namely quantification of the melamine
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concentration in milk with a detection limit of ~ 5 µM.129 Figure 4b also shows graphitic
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nanocapsules, in which the stable and unique Raman signal of graphene is used as internal
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standard.133 The RSD% of SERS intensity of Rhodamine B (RhB) can be reduced from 20.4% to
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6.8%. Sometimes, the signal of the target molecule itself can also be used as a stable intrinsic
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internal standard,134–136 such as the Raman peaks of the phosphate backbone of DNA135 and
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tryptophan of proteins (Figure 4c).136 Accordingly, with the assistance of a proper internal
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standard, it is possible to obtain reliable SERS signals from randomly aggregated nanoparticles
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for quantitative analysis, but requiring careful and rational design of the experiment.
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Highly Ordered Rigid SERS Substrates. With the rapid advancement of nanofabrication
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technology, researchers are able to fabricate SERS-active substrates with ultrafine features and
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large-area uniformity, therefore ensuring a good reproducibility for SERS signals. These
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substrates often contain a hotspot-rich metallic film that is supported by a rigid substrate. For
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more details on the fabrication methods for these SERS substrates, the reader can refer to the
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previously published review.137 In general, these approaches can be categorized as top-down and
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bottom-up techniques. Top-down techniques, such as electron beam (E-beam) lithography,
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focused-ion beam and photon lithography, nano-indentation, and metal-induced chemical etching,
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can fabricate well-controlled nanoscale patterns, and they are promising techniques for
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fabricating SERS-active substrates (Figure 5a).138–146 These approaches have difficulty in
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fabricating sub-5-nm structures (the size of a typical SERS hotspot is approximately 2 nm),
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however, the main challenges of top-down nanotechnology lie in the high technique barrier
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involved, low fabrication speed, and extremely high production cost, limiting their practical
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applications.
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Compared to the top-down approaches, the bottom-up techniques to fabricate ordered
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plasmonic nanostructures are often simpler and more cost-effective (Figure 5b). Usually, this
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approach is realized by assembling smaller nanoparticles into an ordered nanostructures array.
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The wildly used assembly techniques include Langmuir-Blodgett,147 Langmuir-Schaefer,148 gel
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trapping,149 electrophoretic deposition,150 etc. Recently, an interesting approach has been the
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liquid/liquid interface (LLI) or liquid/air interface (LAI) self-assembly technique in which the
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nanoparticle array is assembled on the interface and subsequently transferred to a solid
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supporter.114,151–155 However, substrates fabricated via the bottom-up approaches can only
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achieve moderate reproducibility and batch-to-batch reproducibility. Compared to the top-down
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approaches, these approaches are of a low degree of freedom and are unable to fabricate any
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user-defined structure; therefore, the precise control of substrate shape, distribution, and density
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is impossible.
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Another method for assembling highly-ordered SERS substrates is template-assisted
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fabrication (Figure 5c). The templates include anodized aluminum oxide (AAO),156,157 zinc oxide
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nanowires,158,159 carbon nanotubes,160 and imprinted polymers,161 etc.162–164 Among them, the
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AAO template is extensively used. SERS substrates prepared via the template-assisted method
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are characterized by low cost, high reproducibility, high throughput, and low interference from
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the background signal. In all, compared with nanoparticle colloids, rigid SERS substrates often
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exhibit far better signal reproducibility and sensitivity, which can serve as a solid foundation for
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SERS quantification applications.
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Shell-Isolated Substrates. An irreproducible SERS measurement could also come from a
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physically/chemically unstable SERS-active substrate. For instance, nanoparticle colloids tend to
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aggregate when their surrounding environment changes.165 Meanwhile, metallic plasmonic
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nanostructures often suffer from oxidation and corrosion, both leading to degradation of the
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plasmonic characteristics and changes in the surface morphology, thereby compromising
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effective generation of SERS. Nevertheless, a fluctuation in the SERS measurement can also be
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caused by the photocarbonization, photobleaching or metal-catalyzed site reactions. Therefore, to
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develop any reliable SERS quantifications, these aspects must be paid attention to besides the
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reproducibility of the substrate itself.
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To improve the poor stability of SERS colloid, a chemically inert shell is coated on the
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nanoparticles. With this shell protection, the nanoparticles show good stability in various
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environments. Until now, there have been a variety of materials, such as SiO2,166,167 Al2O3,168,169
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Fe3O4,170,171 TiO2,172–176 MnO2,177 TiN178 and polymers,179 developed for the shell. A typical
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example is shell-isolated nanoparticles (SHINs),169,180 as shown in Figure 6a. It has been
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confirmed that a mixture of concentrated SHINs with pyridine is stable for more than 240 hours.
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In contrast, the mixture of bare gold nanoparticles completely aggregates after 15 mins (Figure
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6a).181 Based on these stable SERS substrates, a simpler quantification method for melamine in
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milk is developed and the LOD can be achieved as low as 0.03 ppm.182 The above coating
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strategy can also be used to prevent oxidation and corrosion of the rigid SERS substrates. This
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approach also helps to prevent photocarbonization, photobleaching or metal-catalyzed site
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reactions for the isolation of the substrate and analyte; therefore, a more reproducible SERS
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signal can be expected. However, this approach is over delicate. Given the rapid decay of the
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LSPR, the inert layer should be very thin (a couple of nanometers) to avoid significant loss of the
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SERS activity. Otherwise, the coating layer must be extremely uniform since any tiny variation
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in the thickness can cause a huge fluctuation in the SERS signal.166,183 The deposition of a
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uniform ultra-thin layer requires sophisticated experimental skills, meticulous treatments or a
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long reaction time, which are very challenging.
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An alternative approach has been proposed in which instead of coating the surface with an
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ultra-thin and uniform inert shell in a highly controllable manner, a thin film with a uniform
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thickness is directly transferred to the substrate as the passivate layer (Figure 6c). CVD-grown
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monolayer graphene is prefect for such an application since it is a mechanically strong and
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chemical inert atomic monolayer with a uniform thickness and is impenetrable to most gas
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molecules and liquids,184,185 which was also developed for fabricating SHINs (Figure 6b).186–188
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By passivating the SERS substrate with monolayer graphene, the substrate exhibited a
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significantly enhanced physical/chemical stability.189–192 Furthermore, graphene-shielded
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substrates also exhibit some new features (Figure 6c), such as alleviating photo-induced damage
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due to the extremely high thermal conductivity of graphene and improving the sensitivity for
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certain analytes due to graphene’s high affinity to many aromatic molecules and biomolecules
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(for the π-π stacking interactions).192 In particular, graphene exhibits excellent biological
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compatibility. These features make graphene-shielded substrates a promising platform for SERS
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quantification.
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3.2 Strategies to Develop SERS Detection in a Complex Sample Matrix
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SERS is known to be a first layer effect, which is only amplified at distances less than ~ 5 nm.
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When applying SERS technology to real-world detection, competitive adsorption or nonspecific
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fouling becomes another major problem for researchers, which significantly reduces the
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sensitivity and specificity of SERS sensing, especially for quantitative analysis. Although this
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problem can be resolved via a sample pretreatment process, it will certainly prolong the
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processing time and comprise the advantages of SERS as a rapid detection method compared to
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other methods. On other hand, target molecules with a weak affinity to substrates or with small
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cross sections present further difficulties. The gap distances with the hotspots only extend a few
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nanometers (~2 nm); accordingly, it is very difficult for analytes with a low affinity to enter such
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small, confined spaces. Therefore, surface modification of the SERS substrate to improve the
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contact between the analyte or to avoid competitive adsorption is essential for real-world SERS
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applications (Figure 7). A table summarizing the surface modification method is shown in Table
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1.
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Electrostatic and Hydrophobic Interactions. Among the various approaches for improving
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the affinity of analytes to substrates, the simplest one is based on electrostatic and hydrophobic
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interactions. In most cases, the capping agent of common colloidal systems for SERS (i.e., citrate
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or borohydride reduced gold or silver particles) is mainly formed via citrate ions and is thus
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negatively charged. As a consequence, to improve the affinity to most of organic molecules that
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often carry negative charges, the commonly used method is to modify the SERS substrates with a
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monolayer of positive-charged capping molecules (Figure 7a).16,193–198 For example, by
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functionalizing the amino group on the surface, negative-charged methylene blue can be
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selectively separated from the sample matrix containing acid blue and be detected.196 An
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alternative approach is to adjust the solution pH to enhance the affinity of the analyte to SERS
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substrate.199,200 By lowering the suspension pH below the analyte pKa, the surface affinity of
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SERS substrates can be improved because of the consistent electrostatic alignment. For example,
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using this method, limits of detection (LOD) of 3 × 10-9 M and 1.1 × 10-8 M for carbamazepine
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and atrazine, respectively, were obtained. By contrast, the LOD of 4.6 × 10-7 M for atrazine in
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drinking water obtained by non-modified SERS substrate (Table 1).201 This method based on
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adjusting pH (without surface modification) overcomes the drawback of interfering background
312
signal due to the modified molecules. However, uncontrolled aggregation and flocculation of
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metallic colloid at low pH need to be avoided.
314
Furthermore, for analytes with no affinity for gold or silver or are highly hydrophobic, surface
315
functionalization is commonly employed to attract target molecules to the SERS substrate via
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hydrophobic interactions, such as an alkanethiol self-assembled monolayer.202–206 Based on the
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strong Au-S bonds, a monolayer of alkanethiol can be easily assembled on the substrates, in
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order to entrap hydrophobic targets. Compared with unmodified SERS substrate, a lower LOD
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can often be achieved for these modified SERS substrates, as shown in Table 1.206 Another
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interesting surface modification strategy is based on the so called host-guest interactions, in
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which a hydrophobic inner cavity and a hydrophilic outline to entrap the target molecules to the
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inner cavity.207–209 This method has gained special attention owing to its ability to capture a batch
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of small hydrophobic non-polar molecules in complex matrix (Figure 7b). For example, three
324
PCB molecules captured by β-cyclodextrin on the substrate surface is confirmed to be detected
325
simultaneously,207 however, with a compromising LOD compared with decanethiol-modified
326
method,206 as shown in Table 1.The above approaches have been successfully applied to analyze
327
real-world samples, such as blood plasma and serum for clinical diagnostics and contaminant
328
tracing in food samples.
329
As we know, competitive adsorption of other molecules or proteins on targets, which results in
330
an appreciable background noise, is a critical problem for real-world applications. To solve this
331
problem, it is common to modify a mixed self-assembled monolayer (SAMs) of thiol molecules
332
and non-fouling zwitterionic polymer, where the former is to attract or probe the target
333
molecules and the latter is to repel nonspecific fouling from the complex sample matrix (Figure
334
7e).210,211 A modified substrate with a surface composition of 94% N,N-dimethyl-cysteamine-
335
carboxybetaine showed that low nonspecific fouling was obtained.212 Using this modified
336
substrate-based SERS system, several analytes in undiluted plasma could be successfully
337
detected, such as drugs, blood pH, fructose and so on. Herein, it is worthy to note that modified
338
zwitterionic molecules not only effectively resist nonspecific fouling but also need to have very
339
weak Raman activity to avoid interference for an effective Raman signal of the target molecules.
340
Another mixed self-assembled monolayer of thiolazide and phosphocholine was also developed
341
for detecting C-reactive protein in serum, where the thiol molecules acts as a linker of
342
phosphocholine and SERS substrate, and the latter one is to attract the target.213 Using this SERS
343
platform, C-reaction protein could be selectively detected with a lower LOD of 10-10 g/mL
344
compared to that obtained by conventional Raman method (Table 1).214 For detecting Sudan dyes
345
in food samples, the Cialla-May group developed a SERS substrate modified with aliphatic
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hydrocarbons to repel non-specific fouling from water-soluble components.215 Via this
347
hydrophobic surface modification, 9 µM Sudan III in paprika powder extract was successfully
348
detected in the presence of riboflavin as a water-soluble competitor. Besides, this lipophilic
349
SERS substrate was also successfully applied to lipids analysis which need lower sampling
350
volume compared to that of Raman technique.216
351
However, surface coatings often lead to a decrease in the SERS signal intensity due to the
352
separation of the analyte and the hotspot in substrates, and noisy background due to the pre-
353
existing modification layer before the detection that may cause difficulties in interpreting the
354
signal or poor signal-to-noise (S/N) ratios.203 More importantly, the specificity of modification
355
layer based on electrostatic and hydrophobic interactions is not very high.203
356
Specific Molecular Receptors. To improve the specific affinity of a SERS substrate to an
357
analyte, molecular receptors have also been employed to modify the substrate surface. Due to the
358
small size of the hotspot, commonly used large molecular receptors in bio/chemical sensing,
359
such as antigen/antibodies, biotin/avidin, aptamers, etc., are not well suited in this case.
360
Therefore, small molecules are usually preferred as the specific molecular receptors.217,218 For
361
example, SERS substrates modified with 4-mercaptophenylboronic acid are widely used for
362
glucose sensing via reversible boronate ester formation.219–226 A triosmium carbonyl cluster-
363
boronic acid conjugate functionalized SERS substrate was successfully used to detect glucose
364
with concentrations as low as 10-4 M (Figure 7c).219 Compared with other methods or SERS
365
substrate without surface modification, specific molecular receptors offer the lower detection
366
limits and simpler operation procedure (Table 1).227 Another typical example is using the proper
367
molecules with both sensitive pH response and strong Raman signals for in vivo SERS-based pH
368
sensing. The most commonly used probe molecules are 4-mercaptobenzoic acid228–230 and 4-
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mercaptopyridine.231
370
mercaptoethanesulfonate exhibit high selectivity for detecting alkaline and alkaline earth metal
371
cations.217 This method depends on the formation of contact-ion pairs between sulfonate groups
372
of 2-mercaptoethanesulfonate and metal cations. A typically low LOD was obtained by this
373
method, e.g., the LOD for Ca2+ was 10-8 M (Table 1). Thus, with the assistance of specific
374
molecular receptors, molecules with weak affinity to SERS substrates or Raman-inactive ions
375
could be detected.
Additionally,
Ag
nanoparticles
functionalized
with
2-
376
Mechanical Trapping. The mechanical trapping of analytes is achieved by stimuli-responsive
377
SERS substrates. The most commonly used stimuli are pH,232,233 temperature234–236 and light.237
378
For example, Au nanorod-doped poly(N-isopropylacrylamide) microgels were presented and
379
applied for SERS application.236 It was demonstrated that the poly(N-isopropylacrylamide)
380
microgels could collapse when heating the temperature beyond 32 ºC; thus, the distance between
381
the Au nanorods could be shortened correspondingly to generate the hotspots. When the
382
temperature cooled down to 32 ºC, the responsive matrix could recover to expanding status
383
(Figure 7d). Therefore, analytes could be trapped in the hotspots by adjusting the temperature.
384
Because of this, the sensitivity of this SERS substrate could be significantly improved compared
385
to conventional SERS substrate (Table 1).238 In addition, dual stimuli-responsive microgels
386
modified SERS substrates were also developed recently. For example, the hybrid microgels were
387
confirmed to be utilized as for measuring both pH value and the temperature of their
388
surroundings.239,240 Another alternative approach is molecularly imprinted polymer-based SERS
389
detection, which usually consists of an ultrathin polymer layer imprinted with the
390
targets.16,161,241,242 This method has been used in various fields, especially for biomolecules
391
analysis (Table 1).243 This method was also successfully applied to selectively adsorb and
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separate α-tocopherol from vegetable oils.244 Nevertheless, the enlarged distance between the
393
analytes and surface of SERS substrates due to the polymer structure results in a modest SERS
394
enhancement.
395
3.3 Strategies to Develop User-Friendly Real-Life SERS Applications
396
With the significant progress that has been made in solving the above fundamental problems
397
such as poor signal reproducibility and low selectivity towards real samples, the SERS technique
398
has thus reached the stage of developing real-life applications. In terms of practical applications,
399
cost is a primary concern, and a good method should above all be an affordable one. However,
400
most of the nanofabricated SERS substrates do not satisfy this prerequisite. Another concern is
401
the convenience and easy usability of the application. For example, the above-mentioned SERS
402
substrates, nanoparticle colloids and rigid substrates, can be conveniently used to analyze liquid
403
forms of samples, but this is much more difficult in the quantification of a target molecule on
404
roughed, irregular, and non-planer surfaces, which is the most common case in real-life
405
applications. Extra extraction processes are thus often required.
406
In-Situ Synthesized SERS Substrates. To meet the requirement of economic viability and
407
operational simplicity, in situ synthesis of SERS substrates has been developed. For example,
408
combined with conductive ink, traditional pens (e.g., fountain pens and ball pens) can be
409
employed to write conductive patterns on multiple substrates, such as glass245 or paper (Figure
410
8a).246,247 Using inkjet technology, micro-volumes of metallic nanoparticle colloids have been
411
directly sprayed onto the surface of artwork and textile fibers, which have been employed to in
412
situ identify the organic colorants of Japanese woodblock print dating to the end of the 19th
413
century (Figure 8a).248 It is worth mentioning that the former one, so-called “pen-on-paper”
414
approach, can obtain a lower LOD of thiabendazole of 20 ppb.247 Similarly, this assay was also
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applied to quantitatively determine melamine in milk with the LOD of 0.27 mg/L.249 Another in
416
situ synthesis approach is to directly immobilize the plasmonic nanoparticles on the sample
417
surface (e.g., bacteria cell wall20,250). The SERS signal of an analyte, e.g., bacteria, using this
418
strategy is approximately 30-fold higher than in the case of a simply mixed colloid-analyte
419
suspension and a lower LOD of 103 CFU/mL of L. innocua can be obtained.250 These methods
420
are relatively low-cost, fast and user-friendly, which have recently led to substantial research
421
interest. However, the poor reproducibility of the Raman signal due to the irregularity of sample
422
surfaces still remains a problem.
423
Flexible SERS Substrates. A flexible SERS substrate refers to a SERS-active plasmonic
424
nanostructure constructed on a flexible solid supporter. Such substrates exhibit several
425
advantages over conventional rigid substrates in terms of easy usability and fabrication cost.
426
Unlike the intrinsic rigid, fragile SERS substrates, these substrates can be attached to rough,
427
irregular (i.e., non-planar or ripply) surfaces and directly collect samples, offering a non-invasive
428
or minimally invasive method of sample analysis. This advantage is especially useful for
429
analyzing fragile and valuable samples. Examples of flexible SERS substrates include cellulose
430
papers,251–254 polymers,255–257 graphene,258 etc (Figure 8b). Our group has also developed a
431
flexible patterned plasmonic metafilm by combining bottom-up self-assembly and top-down
432
laser engraving. With tunable plasmonic properties and excellent flexibility, this plasmonic
433
material could be applied in various fields.259 Nevertheless, flexible SERS can be fabricated via a
434
simple and cost-efficient approach, such as the dip-coating method,252 printing technology,260 or
435
interface self-assembly technique.261 An excellent review on flexible SERS substrates can be
436
referred to for more details.262 The characteristics afforded by a flexible supporter enable flexible
437
SERS substrates several new functionalities and extend to applications that were not possible
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before. For example, flexible SERS substrates can be easily cut into arbitrary shapes and
439
integrated with other devices, such as for capillary-actuated fluid transport and selective
440
molecular retention. Flexible SERS substrates can integrate the sample collection and the SERS
441
detection into one, which is very convenient for in-situ analysis.263,264 It offers other attractive
442
advantages, such as being easy to transport or stable for storage. All of these advantages are
443
expected to bring SERS technology closer to real-world applications.
444
Currently, several groups have used the flexible SERS substrates to analyze chemicals in
445
agricultural products and food. For example, via a simple “paste and peel off” approach,
446
pesticide residues (e.g. parathion-methyl, thiram and chlorpyrifos) in fruits and vegetables could
447
be directly extracted to the as-prepared flexible SERS substrates and analyzed.256,265 The
448
sensitivity of this approach can achieve a few nanogram per square centimeter (ng/cm2).
449
Moreover, using a Ag decorated sandpaper substrate, the triazophos on fruit surface can be
450
directly detected by swabbing the sample surface.266 The LOD can be achieved as low as 4.2
451
pM/cm2. Although flexible substrates can be easily attached on the irregular surface, limited by
452
their poor optical transparency, additional sample exaction procedures (e.g. paste and peel off or
453
swab256,265,266) are always needed for the detection. Therefore, in-situ analysis of residues on
454
sample surface cannot be realized by these opaque SERS substrates.
455
“All Tasks” SERS Substrates. The in situ quantification of chemical molecules on an
456
ambient surface without any sample pretreatment has been a long-standing pursuit in analytical
457
science. Among the few optional techniques, SERS is particularly attractive because of its ultra-
458
high sensitivity and selectivity. However, the most commonly used substrates, such as
459
nanoparticle colloids or rigid substrates, cannot accomplish this task because nanoparticle
460
colloids can hardly generate a reproducible signal over large areas and because rigid substrates
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461
cannot be attached to irregular surfaces. Additionally, conventional flexible substrates due to
462
their poor optical transparency can completely block the signal after attaching to sample surfaces.
463
Thus, the invention of flexible and transparent SERS-active substrates overcomes these
464
dilemmas and can make the idea of in situ SERS detection a reality.
465
Recently, various flexible substrates with transparent properties have been fabricated.261,267–273
466
There are two critical factors for the preparation of transparent flexible SERS substrates. Not
467
only the flexible supporter must be transparent, the SERS-active metallic layer should also
468
exhibit good optical transparency, which is difficulty since metallic layer is often not. A possible
469
approach is using top-down method to fabricate thin metal nanostructures on a transparent
470
polymer film, but is limited by its high cost and low hotspot density.274 The commonly used
471
method is to transfer an as-prepared nanoparticle monolayer array onto a polymer thin film. Our
472
group also introduced a flexible and transparent SERS metafilm.275 A large-area 2D array of
473
silver nanocubes (Ag NCs) was assembled via the LAI approach and then transferred onto
474
cellophane adhesive tape. The obtained Ag NCs array-attached tape exhibited excellent SERS
475
activity and a homogeneous enhancement factor; more importantly, its high mechanical
476
flexibility and good transparency ensure its conformal contact with sample surfaces and enable
477
excitation and collection of the signal from the backside of the substrate. Because of this feature,
478
flexible substrates with transparent properties has become a promising method for in situ
479
analysis of real-world samples. Recently, several groups have employed the flexible and
480
transparent SERS substrate to direct analysis of chemicals in agricultural products and foods,
481
such as pesticide residues and food additives (Figure 8c).261,268–270,276–279 By simply covering
482
these substrates on the surface of samples, the contaminants can be directly detected. For
483
example, pesticide residues can be in situ detected by attaching these substrates to the sample
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surfaces (e.g. fruit, fish). The sensitivity via this approach for detecting thiram on fruit peel and
485
methylene-blue on fish surface can be down to 72 ng/cm2 and 10-13 M, respectively.268,273
486
Besides foods and agricultural products, residues on food packages such as malachite green on
487
the inner wall of container can also be rapidly determined by transparent and flexible SERS
488
substrates.275
489
Therefore, flexible and transparent SERS substrates could serve as a form of “all task”
490
substrates since they suit all forms of samples, such as liquids, gases, solids, and especially
491
analytes dispersed on an underlying nonplanar solid surface.275 It is far more suitable than other
492
substrates for real-world applications, such as in homeland security, forensic science,
493
environmental analysis, food safety, etc. Altogether, these features suggest the high versatility of
494
flexible and transparent SERS-active substrates.
495
Integrated Portable SERS Sensing Devices. To enable SERS technology to be applied to on
496
field detection, portability is another important factor to be concerned. Most of the conventional
497
techniques commonly require sophisticated sampling and separation procedures, which hamper
498
their use in on field analysis. Recently, a variety of portable SERS sensing devices are developed,
499
such as microfluidic chip or device,236,280–284 capillary or microcolumn,12,245,285,286 SERS enabled
500
micropipette,287 paper-based SERS chip,263,288–290 SERS swab,291,292 lab-on-chip device,293 SERS
501
test kit (Figure 8d).294 These on-site SERS platforms usually combine with portable Raman
502
spectrometers which usually integrate separation, concentration and quantification process all in
503
one. For example, a SERS-enabled micropipette integrated with microsampling device was
504
developed to on-site capture and detect surface organic residues on real-world samples. By
505
dropping a proper extraction agent on sample surface, several pesticide residues (thiram,
506
malachite green and methyl parathion) on vegetable surface could be simultaneously detected.
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507
The LOD obtained via this device can be down to 8 nM, 8 nM, 1.5 µM for thiram, malachite
508
green and methyl parathion, respectively.287 Another SERS-enabled cotton swab was also
509
fabricated for sensing surface residues. By simply swabbing the sample surfaces, the targeting
510
contaminants could be collected and then detected. For example, 10-5 M carbaryl on a cucumber
511
surface could be successfully discriminated.291 Many materials, such as glass capillary, paper,
512
micropipette, are employed to fabricate these devices which are cost-effective and easy to obtain.
513
Thus, these portable and low-cost SERS-enabled devices make on-field application come true.
514
4. Perspectives
515
Since its discovery, a tremendous amount of work has been invested in the field of SERS,
516
which has successfully promoted our theoretical and experimental understanding of SERS.
517
Especially with recent breakthroughs in nanotechnology, researchers have gained the ability to
518
fabricate rationally designed and uniform plasmonic nanostructures as high-performance SERS
519
substrates, extremely expanding the potential applications of SERS in various fields. However,
520
most of the applications have, thus far, been limited to solving problems in the laboratory. To
521
further expand the real-life applications of SERS, several challenges need to be addressed. This
522
review attempted to emphasize these crucial but often overlooked problems and summarize
523
possible solutions. We hope that our discussion will encourage more research to address these
524
challenges and eventually help to bring SERS technology out of the laboratory.
525
The success of SERS application relies on the development of SERS-active substrates.
526
Although SERS has been famous for its poor reproducibility, currently, reliable SERS
527
applications can be developed based on highly uniform SERS substrates fabricated via top-down
528
nanofabrication approaches, such as E-beam lithography. The real reason for restraining SERS to
529
the laboratory is the high fabrication cost, which is one of the primary concerns for developing
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530
real-life applications. Therefore, the further expansion of SERS depends on developing cost-
531
efficient and high-performance substrates. Several possible strategies to address this challenge
532
are listed above, either by improving conventional SERS colloids or by developing low-cost
533
assembly techniques to fabricate uniform substrates. We can expect to see some practical SERS
534
applications based on these advances in the near future.
535
The design of SERS substrates used to focus on SERS-active nanostructures to achieve a
536
higher EF and better uniformity, but what is often neglected is the affinity of the substrates to the
537
analytes, which is a prerequisite for obtaining a reliable and sensitive SERS signal. Moreover,
538
nonspecific fouling of a SERS substrate in a real sample matrix and weak affinity of the analytes
539
to SERS-active substrates are the two main problems that need to be solved. Thus, how to
540
combine a proper molecular capture strategy (such as preconcentration methods,295–297 aptamer
541
recognition298) with repelling fouling approaches to improve the sensitivity and selectivity of
542
SERS detection is still an important issue for the future. A more delicate design approach should
543
be carried out.
544
One of the most promising and practicable application of SERS is the rapid testing for
545
chemical residues in foods or agricultural products, such as pesticide. By simply spreading the
546
SERS colloids on sample surface or swabbing the sample surface with a soft SERS substrates,
547
the residues could be rapidly analyzed.299–301 Besides, various user-friendly SERS sensing
548
devices have been developed, such as microfluidic device,236 capillary,285,286 SERS enabled
549
micropipette,287 paper-based SERS test strip,289 and SERS swab,291 which were rationally
550
designed for this purpose. The entire process, including preconcentration, separation and SERS
551
detection, can be completed within a few mintutes.299,302 Furthermore, recent development in
552
transparent flexible SERS substrates can even allow analysis of chemical residues directly on the
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553
sample surface, by directly attaching this substrate to the sample surface and recording SERS
554
signal from its back.303 These advances will definitely benefit to bring SERS technology closer
555
to real-world applications.
556
Further Challenges.
557
The mismatch between the size of the hotspot and that of the target is one of the main
558
problems for SERS detection of biomolecules. Thus, we can obtain only a partial SERS signal of
559
biomolecules; therefore, a fluctuation in the SERS signal is expected due to intrinsic distance-
560
dependent effects of SERS and the relatively large and complex structures of the biomolecules.
561
Several approaches have been employed to overcome this problem, such as surface modification,
562
which aimed to slightly enlarge the distance between analytes and substrates to ensure
563
enhancement of the absolute biomolecule signal.135,136 However, how to sensitively and
564
selectively detect biomolecules using SERS still remains a challenge.
565
Furthermore, to shorten the distance between laboratory research and real applications,
566
portable Raman devices need to be considered. For example, a smartphone-based point-of-care
567
genetic testing device that is inexpensive, user-friendly and compact has been developed.304 And
568
a lot of mobile Raman spectrometers have also been developed in various on-site SERS
569
applications.305 Combining rationally designed SERS-active substrates with portable Raman
570
spectrometer, these SERS-enabled platforms, usually integrated separation, concentration and
571
quantification processes all in one, are worth for future research.
572 573
Acknowledgements
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This work was financially supported by the National Natural Science Foundation of China (No.
575
21305125), the Natural Sciences Fund of Zhejiang Province (No. LY14B050004) and Zhejiang
576
Provincial Public Welfare Technology Applied Research Project (No. 2016C32006).
577
Notes
578
The authors declare no competing financial interest.
579
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580 581 582 583
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1420 1421 1422
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1423 1424 1425
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dipicolinic acid, an anthrax biomarker, considerably below the infective dose. Chem. Commun. 2016, 52 (64), 9925–9928.
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FIGURE CAPTIONS
1460
Figure 1. Working principle of SERS. (a) Schematic diagram of the targets on the surface of
1461
SERS substrates under light excitation. (b) Illustration of the localized surface plasmon
1462
resonance effect; (c) Finite-element simulations of SERS enhancement distribution in single and
1463
coupled Au nanostructures. Copyright with the permission from ref.47,48,50.
1464
Figure 2. Key developing stages of SERS-active substrates. Copyright with the permission from
1465
ref.62,64,65,68,69,140,143,146,159,164,178,252,257,259,271,275.
1466
Figure 3. Comparation of random aggregation and controllable assembly of metallic colloid
1467
substrates. (a) Random SERS-active nanostructures. (b) DNA motif-guided assembly. (c)
1468
Liquid/liquid interfacial assembly. Copyright with the permission from ref.72,73,76,77,90,93,95–
1469
98,100,102,106,108,112,114,119
1470
Figure 4. New types of internal standards for quantitative SERS detection. (a) Extrinsic internal
1471
standards (core-shell structure). (b) Extrinsic internal standards (Graphitic internal standard). (c)
1472
Intrinsic internal standards. Copyright with the permission from ref.125,126,133,135.
1473
Figure 5. Highly ordered rigid SERS substrates fabricated by different methods. (a) Top-down
1474
fabrication. (b) Bottom-up assembly. (c) Template-assisted fabrication. Copyright with the
1475
permission from ref.114,138,139,141–145,148,149,151,153,155,157–159,162–164.
1476
Figure 6. Shell-isolated SERS substrates. (a) Traditional shell-isolated nanoparticle. (b)
1477
Graphene-encapsulated nanoparticle. (c) Graphene-isolated substrate. Copyright with the
1478
permission from ref.180,181,186,187,192.
.
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Figure 7. Strategies to develop SERS detection in complex sample matrix. (a) Electrostatic
1480
interaction. (b) Hydrophobic effect (Host-guest interaction). (c) Specific molecular receptors. (d)
1481
Mechanical trapping. (e) Mixed self-assembly monolayers (SAMs). Copyright with the
1482
permission from ref.197,209,211,219,235.
1483
Figure 8. User-friendly SERS substrates. (a) In situ synthesized SERS substrates. (b) Low cost
1484
flexible SERS substrates. (c) Universal flexible substrates. (d) Integrated portable SERS devices.
1485
Copyright with the permission from ref.236,247,248,254–256,258–260,268,269,276,283,287,292.
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TABLE CAPTIONS
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Table 1 Comparation of SERS platforms with or without surface functionalization.
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Figure 1.
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Figure 2
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Figure 4
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Figure 6
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Table 1 strategy electrostatic interaction
N.A.
modified molecule
hydrophobic interaction
decanethiol
hydrophobic interaction
β-cyclodextrin
target atrazine poly-chlorinated biphenyls-77 poly-chlorinated biphenyls(-3, -29, -77)
LODb 4.6 × 10-7 M 201
10-7 M
10-6 M
206
10-6 M
N.A.
207
3 × 10-5 g/mL214
213
N.A. 5 × 10-3 M 227
217
N.A.
231
N.A.
228
~10-5 M 238
236
10-5 M 243
161
thiolazide C-reactive protein 10-10 g/mL phosphocholine 2+ 10-8 M molecular receptor 2-mercaptoethanesulfonate Ca molecular receptor 4-mercaptophenylboronic acid glucose 10-4 M 4-mercaptopyridine molecular receptor pH 4.0-9.0 bull serum albumin 4-mercaptobenzoic acid molecular receptor pH 4.5-8.5 peptide poly(N-isopropylacrylamide) mechanical trapping methyl parathion 10–7 M microgel molecularly imprinted polymers mechanical trapping transferrin 10-8 M (dopamine) a : Except the result for pH (pH response range), the results refer to the LOD of the targets. b : LOD obtained by a SERS substrate without surface modification. hydrophobic interaction
1542 1543 1544
resultsa 1.1 × 10-8 M
ref. 200
219
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