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Facing Challenges in Real-Life Application of Surface-Enhanced Raman Scattering: Design and Nanofabrication of Surface-Enhanced Raman Scattering Substrates for Rapid Field Test of Food Contaminants Ruyi Shi,† Xiangjiang Liu,† and Yibin Ying*,†,‡

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†

College of Biosystems Engineering and Food Science, Zhejiang University, 866 Yuhangtang Road, Hangzhou, Zhejiang 310058, China ‡ Zhejiang A&F University, 88 Huanchengdong Road, Hangzhou, Zhejiang 311300, China ABSTRACT: Surface-enhanced Raman scattering (SERS) is capable of detecting a single molecule with high specificity and has become a promising technique for rapid chemical analysis of agricultural products and foods. With a deeper understanding of the SERS effect and advances in nanofabrication technology, SERS is now on the edge of going out of the laboratory and becoming a sophisticated analytical tool to fulfill various real-world tasks. This review focuses on the challenges that SERS has met in this progress, such as how to obtain a reliable SERS signal, improve the sensitivity and specificity in a complex sample matrix, develop simple and user-friendly practical sensing approach, reduce the running cost, etc. This review highlights the new thoughts on design and nanofabrication of SERS-active substrates for solving these challenges and introduces the recent advances of SERS applications in this area. We hope that our discussion will encourage more researches to address these challenges and eventually help to bring SERS technology out of the laboratory. KEYWORDS: SERS, SERS-active substrate, nanofabrication, food contaminant, rapid field test

1. INTRODUCTION Surface-enhanced Raman scattering (SERS) was first observed in 1974 by Fleischmann when pyridine was adsorbed on a roughened silver electrode1 and was correctly interpreted later by Van Duyne and Creighton in 1977.2,3 Since its discovery, SERS remained interesting only to a relatively small scientific community, e.g., to the electrochemistry or Raman spectroscopy fields. It was not until the discovery in 1997 that SERS can reach the single molecule detection limit,4,5 that it suddenly attracted enormous attention. Currently, because of the high sensitivity and high specificity inherited from the unique molecular spectral fingerprints of the Raman spectrum, a large range of disciplines (including chemistry,6,7 physics,8 materialogy,9 and life sciences10) have begun to exploit the potential of this technique. Specifically, in analysis of foods and agricultural products, SERS technique has been used as a rapid and sensitive tool for detecting chemical and microbial hazards. Compared with the well-established “wet chemistry” methods and modern analytical instrumental techniques (HPLC, GC-MS, etc.), SERS can avoid complicated and time-consuming sample pretreatment, therefore, being more preferred in rapid field screening. Another advantage of SERS is that it has less interference from water, which is very important in analyzing biological samples such as agricultural products. Various food contaminants, such as pesticide,11−13 illegal food additives,14,15 veterinary drug,16,17 toxins,18,19 have been successfully detected by SERS technique. Besides, SERS has also been developed as a rapid diagnostic tool for food-borne pathogenic microorganisms.20,21 Therefore, SERS shows great promise in rapid safety assessment of foods and agricultural products. © 2017 American Chemical Society

Several excellent review articles related to SERS have been recently published. Most reviews focus on the mechanism of the SERS effect,22−24 applications of SERS in various fields,25−33 fabrication of SERS-active substrates,34−36 and advances in new SERS devices,37−41 which are of interest to a heterogeneous readership. Naturally, this review cannot cover the entire picture of relevant topics but instead presents the reader with the recent advances of SERS from a different angle. With a deeper understanding of the SERS effect and advances in nanofabrication technique, SERS is now on the edge of going out of the laboratory and becoming a sophisticated analytical tool to fulfill various real-world tasks. This review focuses on the major challenges that SERS has met in this progress, such as how to obtain a reliable SERS signal, how to reduce the running cost of this technology, etc., which are the keys to developing any practical applications. Specifically, in analysis real-world samples (e.g., fruits, vegetables, meats, oils, beverages, etc.), how to increase specificity and avoid interferences from a complex sample matrix and how to simplify the sampling and practicing approach have become especially important. This review highlights the new thoughts on design and nanofabrication of SERS-active substrates for solving these challenges and recent Special Issue: Nanotechnology Applications and Implications of Agrochemicals toward Sustainable Agriculture and Food Systems Received: Revised: Accepted: Published: 6525

July 6, 2017 September 4, 2017 September 18, 2017 September 18, 2017 DOI: 10.1021/acs.jafc.7b03075 J. Agric. Food Chem. 2018, 66, 6525−6543

Review

Journal of Agricultural and Food Chemistry

Figure 1. Working principle of SERS: (a) schematic diagram of the targets on the surface of SERS substrates under light excitation, (b) illustration of the localized surface plasmon resonance effect, and (c) finite-element simulations of SERS enhancement distribution in single and coupled Au nanostructures. Copyright with the permission from refs 47 (Copyright 2015 American Chemical Society), 48 (Copyright 2015 American Chemical Society), and 50 (Copyright 2016 Macmillan Publishers Ltd.).

adsorbed molecules on the substrate relative to the normal Raman intensity of the same molecule using the following equation,

applications of SERS in the analysis of agricultural products and foods as well as the future developments and several prospects of SERS.

2. BRIEF INTRODUCTION OF SERS It has been generally accepted that two enhancement mechanisms may contribute to the enormous enhancement of SERS: an electromagnetic (EM) enhancement42−44 and a chemical enhancement.45 The former one is considered as the dominant reason for SERS,46 which originates from collectively oscillating conduction electrons in metal or metal-like nanomaterials under light excitation (see Figure 1a,b), the so-called surface plasmon resonance (SPR), including localized surface plasmon resonance (LSPR) and propagating surface plasmons (PSP).47−50 This effect can focus the incident light to nanoscale edges, gaps, tips, or crevices, therefore creating many electromagnetic “hotspots” in these sites, whose intensity can achieve 2−5 orders of magnitude of the incident light (Figure 1c).51,52 Molecules trapped in these hotspots can experience an enormous enhancement of their Raman intensity.4 Therefore, it would be more appropriate to call this effect plasmon-enhanced Raman scattering (PERS).50 However, we still use the common term, SERS, in this review, considering its long history and broad acceptance. For interested readers, these fundamental aspects of SERS can be referred to the review by Schlücker.53 These metallic plasmonic nanostructures, also referred to as SERS-active substrates, are the prerequisite for observing SERS. The performance of a SERS-active substrate is usually evaluated based on two aspects: the enhancement factor (EF) and signal uniformity. Generally, the EF of a SERS substrate determines how sensitively it can detect a certain analyte. According to EM enhmacement mechanism, the EF is approximated to be proportional to the fourth power of the enhancement of the local electromagnetic field (|E/E0|4), where E and E0 are the intensities of the local electromagnetic field in the presence and absence of the substrates. The experimental determination of EFs requires measurements of the SERS intensity for the

EF =

ISERS/NSERS IRS/NRS

(1)

where ISERS is the total SERS intensity and IRS is the normal Raman intensity of the probe molecules. NSERS and NRS are the numbers of molecules contributing under SERS and normal Raman conditions, respectively. The EF in the hotspot can be as high as 1014, which is sufficient for single-molecule detection.4,5 However, in most cases, the average EF of the entire SERS substrate is approximately 105−108.43,46,49 To further increase the EF of a SERS substrate requires a rational design and precise fabrication. However, it is a formidable task since so many factors need to be considered, such as the incident laser wavelength, optical properties of the analyte, the material of the substrate and, most importantly, the morphology of the metallic plasmonic nanostructures of the substrate.54 The signal uniformity of a SERS substrate is another crucial parameter since it decides how reliable a SERS measurement can be, which is the key to quantitative analysis. Evaluation of the reproducibility of SERS-active substrates is usually carried out by the relative standard deviation (RSD) value of the SERS intensity obtained on randomly selected spots. However, the complexity of evaluation the EF and the signal uniformity of a substrate should not be underestimated since several factors need to be taken into consideration together,55,56 such as the excitation wavelength and LSPR modes, the choice of probe molecule (avoiding resonance Raman effect), determination of the coverage of the probe on the substrates, etc. A full consideration of these factors should be given before comparing different SERS substrates. SERS-active substrates are the foundation for any SERS applications. An ideal SERS-active substrate generally needs to 6526

DOI: 10.1021/acs.jafc.7b03075 J. Agric. Food Chem. 2018, 66, 6525−6543

Review


Figure 2. Key developing stages of SERS-active substrates. Copyright with the permission from refs 61 (Copyright 2009 American Chemical Society), 63 (Copyright 2013 Elsevier), 65 (Copyright 2002 American Association for the Advancement of Science), 68 (Copyright 2015 Elsevier), 69 (Copyright 2016 Elsevier), 140 (Copyright 2013 Macmillan Publishers Ltd.), 143 (Copyright 2012 American Chemical Society), 146 (Copyright 2015 Royal Society of Chemistry), 159 (Copyright 2010 American Chemical Society), 164 (Copyright 2015 Wiley), 176 (Copyright 2017 American Chemical Society), 250 (Copyright 2015 Royal Society of Chemistry), 255 (Copyright 2017 Royal Society of Chemistry), 257 (Copyright 2016 Wiley), 269 (Copyright 2016 Royal Society of Chemistry), and 273 (Copyright 2016 American Chemical Society).

Figure 3. Comparation of random aggregation and controllable assembly of metallic colloid substrates: (a) random SERS-active nanostructures, (b) DNA motif-guided assembly, and (c) liquid/liquid interfacial assembly. Copyright with the permission from refs 72 (Copyright 2014 American Chemical Society), 73 (Copyright 2014 Royal Society of Chemistry), 76 (Copyright 2017 Springer), 77 (Copyright 2015 Royal Society of Chemistry), 90 (Copyright 2011 Macmillan Publishers Ltd.), 93 (Copyright 2014 Macmillan Publishers Ltd.), 95 (Copyright 2015 American Chemical Society), 96 (Copyright 2015 American Chemical Society), 97 (Copyright 2016 American Chemical Society), 98 (Copyright 2014 American Chemical Society), 100 (Copyright 2013 Macmillan Publishers Ltd.), 102 (Copyright 2009 Macmillan Publishers Ltd.), 106 (Copyright 2013 Macmillan Publishers Ltd.), 108 (Copyright 2016 American Chemical Society), 112 (Copyright 2014 American Chemical Society), 114 (Copyright 2014 American Chemical Society), and 119 (Copyright 2016 Royal Society of Chemistry).

3. CHALLENGES IN REAL-LIFE SERS APPLICATIONS Despite the long history since its discovery, SERS has not yet become a sophisticated tool for practical applications. A major obstacle has been the poor reproducibility and poor stability of the wildly used metallic colloid substrates, limiting its applications in quantitative analysis. Also, when applying SERS technology to real-world applications, the cost is another major concern. Additionally, for practical applications, sample matrix interference cannot be overlooked. Since target molecules that are usually in low concentration and the matrix has the same access to the hotspots, a significant spectral “contamination” can often be observed in a complex sample matrix, which causes difficulty in analyzing the results and

possess ultrafine features, large-scale uniformity, great EF, good signal reproducibility, and a low fabrication cost. Thus far, the development of SERS-active substrates has gone through four stages: (1) metallic colloid substrates, (2) highly reproducible rigid substrates, (3) mechanically flexible substrates, and (4) universal “all-task” substrates (Figure 2). We will later discuss how to use these substrates to deal with the challenges in developing real-life applications and highlight the recent breakthroughs aiming to bring the SERS technique out of the laboratory. 6527


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Figure 4. New types of internal standards for quantitative SERS detection: (a) extrinsic internal standards (core-shell structure), (b) extrinsic internal standards (Graphitic internal standard), and (c) intrinsic internal standards. Copyright with the permission from refs 125 (Copyright 2014 Royal Society of Chemistry), 126 (Copyright 2015 Wiley), 133 (Copyright 2016 Springer), and 135 (Copyright 2015 American Chemical Society).

hotspots generated by aggregation are randomly organized and impossible to control (Figure 3a),75,76 and the aggregated nanoparticles are not stable and tend to sediment under gravity,77 causing both a fluctuation of the strength and number of EM hotspots in the detecting volume. Thus, the obtained SERS signals vary significantly from experiment to experiment and from time to time. It is quite often that the signal variation exceeds 50% using aggregated NPs.75,78 Therefore, it is almost impossible to use them for quantitative analysis. This is the reason that SERS gained its fame for lacking reproducibility. Controllable Assembly. To solve this problem, one of the strategies by using a controllable assembly technique is to synthesize well-defined superstructures based on small nanoparticles. With the help of some ligands, such as organic molecules,79−81 polymers,82 antibody/antigens,83 biotin/avidin connectors,84 and nanoparticles can be assembled into ordered superstructures, such as dimers or trimers,85 core-satellites,86−89 etc. One smart approach is based on the unique base-pairing rules and structural features of DNA, which can be used to program the assembly of ordered plasmonic superstructures,90 such as dimers,91−95 trimers,96 regularly spaced nanoparticles chains,97 and two- or three-dimensional ordered arrays98−104 (Figure 3b). Another interesting approach is the liquid/liquid interfacial assembly technique. On the basis of soft ligand− ligand interactions, including steric hindrance, hydrogen bonding interactions, electrostatic forces,105−107 a variety of morphologies and structures have been successfully assembled, such as nanosheets,108−112 nanoribbons,113 folding structure,114,115 spherical supercluster,116 liquid-like metal droplets,117−120 and so forth (Figure 3c). These densely packed interfacial nanoparticle films have also been developed as SERSactive substrates.106,107 Because of dynamic nanogaps in the

decrease of the sensitivity and specificity. Needless to say, there are many more challenges that need to be addressed before further expanding SERS to real-life applications. 3.1. Strategies to Improve the Signal Reproducibility of SERS. Although the discovery of SERS was made on electrochemically roughened silver electrodes,1 the first type of wildly used SERS-active substrates were metallic nanoparticles, e.g., gold, silver, and copper nanoparticles, freely suspended in a homogeneous medium. Due to the advantages of easy preparation, low cost, and tunable optical properties, these substrates have been preferred for several decades and extensively used in SERS studies. These substrates significantly reduced the technical barrier for performing a SERS experiment and made SERS popular. For the preparation methods of various metallic colloids, the reader can refer to previously published reviews.57,58 Since the EF from single isolated sphere nanoparticle is relatively small, it is often increased by two approaches: either by increasing the intensity or the density of the EM hotspot. Because of the “lightning rod effect”,51 metal nanoparticles with sharp edges and corners exhibit a much higher enhancement of the local EM field compared to the spheres.59 Therefore, various heteromorphic nanoparticles have been employed to improve the EF, such as a triangle,60 polyhedron,61 rod,62,63 cube,64,65 wire,66 star,67,68 popcorn,69 dendrite,70 and so on. Another approach is to increase the density of hotspot in the substrates, by adding salt (e.g., NaCl, KCl, or NaNO3) to trigger the aggregation of nanoparticles. This process generates abundant hotspots between the gaps of aggregated nanoparticles. Aggregation can also be induced by centrifugal force71 and magnetic force.72,73 Sometimes, aggregation is inevitable, as simply introducing the analyte can induce it.74 However, these 6528


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Figure 5. Highly ordered rigid SERS substrates fabricated by different methods: (a) top-down fabrication, (b) bottom-up assembly, and (c) template-assisted fabrication. Copyright with the permission from refs 114 (Copyright 2014 American Chemical Society), 138 (Copyright 2016 American Chemical Society), 139 (Copyright 2012 American Chemical Society), 141 (Copyright 1998 Elsevier), 142 (Copyright 2016 Royal Society of Chemistry), 143 (Copyright 2012 American Chemical Society), 144 (Copyright 2016 Springer), 145 (Copyright 2017 American Chemical Society), 148 (Copyright 2015 American Chemical Society), 149 (Copyright 2004 Royal Society of Chemistry), 151 (Copyright 2016 American Chemical Society), 153 (Copyright 2016 American Chemical Society), 155 (Copyright 2013 Royal Society of Chemistry), 157 (Copyright 2013 Wiley), 158 (Copyright 2017 MDPI), 159 (Copyright 2010 American Chemical Society), 162 (Copyright 2015 American Chemical Society), 163 (Copyright 2009 American Chemical Society), and 164 (Copyright 2015 Wiley).

target molecules for the surface hotspots and will not be influenced by the external environment, providing a stable signal for calibration.125−132 For example, after correction with internal standards, the reproducibility of SERS signals between batches was significantly improved with a RSD% less than 8% (Figure 4a).126 This approach has also been used to develop a real-life application, namely, quantification of the melamine concentration in milk with a detection limit of ∼5 μM.129 Figure 4b also shows graphitic nanocapsules, in which the stable and unique Raman signal of graphene is used as an internal standard.133 The RSD% of SERS intensity of Rhodamine B (RhB) can be reduced from 20.4% to 6.8%. Sometimes, the signal of the target molecule itself can also be used as a stable intrinsic internal standard,134−136 such as the Raman peaks of the phosphate backbone of DNA135 and tryptophan of proteins (Figure 4c).136 Accordingly, with the assistance of a proper internal standard, it is possible to obtain reliable SERS signals from randomly aggregated nanoparticles for quantitative analysis but requiring careful and rational design of the experiment. Highly Ordered Rigid SERS Substrates. With the rapid advancement of nanofabrication technology, researchers are able to fabricate SERS-active substrates with ultrafine features and large-area uniformity, therefore ensuring a good reproducibility for SERS signals. These substrates often contain a hotspot-rich metallic film that is supported by a rigid substrate. For more details on the fabrication methods for these SERS substrates, the reader can refer to the previously published review.137 In general, these approaches can be categorized as top-down and bottom-up techniques. Top-down techniques, such as electron beam (E-beam) lithography, focused-ion beam

liquid interfacial structure, interfacial metal nanoparticle arrays can cause surrounding molecules to easily diffuse into the nanogaps (hotspot).121 In these superstructures, the hotspots are evenly distributed, which ensures the good reproducibility of the signal (RSD% below 10%).121,122 Thus, these interfacial nanoparticle films overcomes the main drawbacks of conventional aggregated colloid substrates. However, the ligands required in the assembly process usually are strongly adsorbed on the nanoparticles and rest in the hotspots, therefore preventing the analyte from entering the hotspots for detection. Removing the ligands often involves harsh treatments and can cause complete or partial disassembly of the superstructures, therefore, limiting their practical applications. Internal Standard. A smart strategy for reducing the fluctuation of SERS intensity caused by uncontrollable aggregation of nanoparticles is to use an internal standard. The internal standard and target molecules normally have similar molecular structures and coexist in the same physical and chemical environment of the hotspot; therefore, its signal can be used to calibrate the absolute signal of each SERS measurement.123 It has been proven that the implementation of an internal standard can reduce the SERS signal fluctuation to the 10% level,124 which is enough for quantitative analysis. However, the reproducibility of this approach could be compromised, due to an unstable internal signal caused by interference from the external environment or competitive adsorption with the analytes, etc. In recent years, several new types of internal standards have been introduced (Figure 4). A special core/shell nanoparticle with internal standards embedded was developed (Figure 4a). Since internal standards are embedded inside the shell, they will not compete with the 6529


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Figure 6. Shell-isolated SERS substrates: (a) traditional shell-isolated nanoparticle, (b) graphene-encapsulated nanoparticle, and (c) grapheneisolated substrate. Copyright with permission from refs 178 (Copyright 2015 Royal Society of Chemistry), 179 (Copyright 2012 Macmillan Publishers Ltd.), 184 (Copyright 2015 Macmillan Publishers Ltd.), 185 (Copyright 2014 American Chemical Society), and 190 (Copyright 2015 Macmillan Publishers Ltd.).

extensively used. SERS substrates prepared via the templateassisted method are characterized by low cost, high reproducibility, high throughput, and low interference from the background signal. In all, compared with nanoparticle colloids, rigid SERS substrates often exhibit far better signal reproducibility and sensitivity, which can serve as a solid foundation for SERS quantification applications. Shell-Isolated Substrates. An irreproducible SERS measurement could also come from a physically/chemically unstable SERS-active substrate. For instance, nanoparticle colloids tend to aggregate when their surrounding environment changes.165 Meanwhile, metallic plasmonic nanostructures often suffer from oxidation and corrosion, both leading to degradation of the plasmonic characteristics and changes in the surface morphology, thereby compromising effective generation of SERS. Nevertheless, a fluctuation in the SERS measurement can also be caused by the photocarbonization, photobleaching, or metalcatalyzed site reactions. Therefore, to develop any reliable SERS quantifications, these aspects must be paid attention to besides the reproducibility of the substrate itself. To improve the poor stability of SERS colloid, a chemically inert shell is coated on the nanoparticles. With this shell protection, the nanoparticles show good stability in various environments. Until now, there have been a variety of materials, such as SiO2,166,167 Al2O3,168,169 Fe3O4,170,171 TiO2,172−174 MnO2,175 TiN,176 and polymers,177 developed for the shell. A typical example is shell-isolated nanoparticles (SHINs),169,178 as shown in Figure 6a. It has been confirmed that a mixture of concentrated SHINs with pyridine is stable for more than 240 h. In contrast, the mixture of bare gold nanoparticles completely aggregates after 15 min (Figure 6a).179 On the basis of these stable SERS substrates, a simpler quantification

and photon lithography, nanoindentation, and metal-induced chemical etching, can fabricate well-controlled nanoscale patterns, and they are promising techniques for fabricating SERS-active substrates (Figure 5a).138−146 These approaches have difficulty in fabricating sub-5 nm structures (the size of a typical SERS hotspot is approximately 2 nm); however, the main challenges of top-down nanotechnology lie in the high technique barrier involved, low fabrication speed, and extremely high production cost, limiting their practical applications. Compared to the top-down approaches, the bottom-up techniques to fabricate ordered plasmonic nanostructures are often simpler and more cost-effective (Figure 5b). Usually, this approach is realized by assembling smaller nanoparticles into an ordered nanostructures array. The wildly used assembly techniques include Langmuir−Blodgett,147 Langmuir−Schaefer,148 gel trapping,149 electrophoretic deposition,150 etc. Recently, an interesting approach has been the liquid/liquid interface (LLI) or liquid/air interface (LAI) self-assembly technique in which the nanoparticle array is assembled on the interface and subsequently transferred to a solid supporter.114,151−155 However, substrates fabricated via the bottom-up approaches can only achieve moderate reproducibility and batch-to-batch reproducibility. Compared to the top-down approaches, these approaches are of a low degree of freedom and are unable to fabricate any user-defined structure; therefore, the precise control of substrate shape, distribution, and density is impossible. Another method for assembling highly ordered SERS substrates is template-assisted fabrication (Figure 5c). The templates include anodized aluminum oxide (AAO),156,157 zinc oxide nanowires,158,159 carbon nanotubes,160 and imprinted polymers,161 etc.162−164 Among them, the AAO template is 6530


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Figure 7. Strategies to develop SERS detection in a complex sample matrix: (a) electrostatic interaction, (b) hydrophobic effect (host−guest interaction), (c) specific molecular receptors, (d) mechanical trapping, and (e) mixed self-assembly monolayers (SAMs). Copyright with the permission from refs 195 (Copyright 2017 Royal Society of Chemistry), 207 (Copyright 2014 Macmillan Publishers Ltd.), 209 (Copyright 2016 Macmillan Publishers Ltd.), 217 (Copyright 2013 American Chemical Society), and 233 (Copyright 2015 American Chemical Society).

Table 1. Comparation of SERS Platforms with or without Surface Functionalization strategy electrostatic interaction hydrophobic interaction hydrophobic interaction hydrophobic interaction molecular receptor molecular receptor molecular receptor molecular receptor mechanical trapping mechanical trapping

modified molecule NA decanethiol β-cyclodextrin thiolazide phosphocholine 2-mercaptoethanesulfonate 4-mercaptophenylboronic acid 4-mercaptopyridine bull serum albumin 4-mercaptobenzoic acid peptide poly(N-isopropylacrylamide) microgel molecularly imprinted polymers (dopamine)

target atrazine polychlorinated biphenyls-77 polychlorinated biphenyls (-3, -29, -77) C-reactive protein Ca2+ glucose pH pH methyl parathion transferrin

resultsa −8

1.1 × 10 M 10−7 M 10−6 M 10−10 g/mL 10−8 M 10−4 M 4.0−9.0 4.5−8.5 10−7 M 10−8 M

LODb

ref

4.6 × 10−7 M199 10−6 M NA 3 × 10−5 g/mL212 NA 5 × 10−3 M225 NA NA ∼10−5 M236 10−5 M241

198 204 205 211 215 217 229 226 234 161

a Except the result for pH (pH response range), the results refer to the LOD of the targets. bLOD obtained by a SERS substrate without surface modification.

substrate with monolayer graphene, the substrate exhibited a significantly enhanced physical/chemical stability.187−190 Furthermore, graphene-shielded substrates also exhibit some new features (Figure 6c), such as alleviating photoinduced damage due to the extremely high thermal conductivity of graphene and improving the sensitivity for certain analytes due to graphene’s high affinity to many aromatic molecules and biomolecules (for the π−π stacking interactions).190 In particular, graphene exhibits excellent biological compatibility. These features make graphene-shielded substrates a promising platform for SERS quantification. 3.2. Strategies to Develop SERS Detection in a Complex Sample Matrix. SERS is known to be a first layer effect, which is only amplified at distances less than ∼5 nm. When applying SERS technology to real-world detection, competitive adsorption or nonspecific fouling becomes another major problem for researchers, which significantly reduces the sensitivity and specificity of SERS sensing, especially for quantitative analysis. Although this problem can be resolved via a sample pretreatment process, it will certainly prolong the processing time and comprise the advantages of SERS as a rapid detection method compared to other methods. On other hand, target molecules with a weak affinity to substrates present further difficulties. The gap distances with the hotspots only

method for melamine in milk is developed and the LOD can be achieved as low as 0.03 ppm.180 The above coating strategy can also be used to prevent oxidation and corrosion of the rigid SERS substrates. This approach also helps to prevent photocarbonization, photobleaching, or metal-catalyzed site reactions for the isolation of the substrate and analyte; therefore, a more reproducible SERS signal can be expected. However, this approach is over delicate. Given the rapid decay of the LSPR, the inert layer should be very thin (a couple of nanometers) to avoid significant loss of the SERS activity. Otherwise, the coating layer must be extremely uniform since any tiny variation in the thickness can cause a huge fluctuation in the SERS signal.166,181 The deposition of a uniform ultrathin layer requires sophisticated experimental skills, meticulous treatments or a long reaction time, which are very challenging. An alternative approach has been proposed in which instead of coating the surface with an ultrathin and uniform inert shell, a thin film with a uniform thickness is directly transferred to the substrate as the passivate layer (Figure 6c). The CVD-grown monolayer graphene is perfect for such an application since it is a mechanically strong and chemical inert atomic monolayer with a uniform thickness and is impenetrable to most gas molecules and liquids,182,183 which was also developed for fabricating SHINs (Figure 6b).184−186 By passivating the SERS 6531


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detected, such as drugs, blood pH, fructose, and so on. Herein, it is worthy to note that modified zwitterionic molecules not only effectively resist nonspecific fouling but also have very weak Raman background. Another mixed self-assembled monolayer of thiolazide and phosphocholine was also developed for detecting C-reactive protein in serum.211 Using this SERS platform, the C-reaction protein could be selectively detected with a lower LOD of 10−10 g/mL (Table 1).212 For detecting Sudan dyes in complex food samples, the Cialla-May group developed a SERS substrate modified with aliphatic hydrocarbons to repel nonspecific fouling from water-soluble components.213 Via this hydrophobic surface modification, 9 μM Sudan III in paprika powder extract was successfully detected in the presence of riboflavin as a water-soluble competitor. Besides, the lipophilic SERS substrate was also successfully applied to lipids analysis which need lower sampling volume compared to that of the Raman technique.214 However, surface coatings often lead to a decrease in the SERS signal intensity due to the separation of the analyte and the hotspot in substrates, and noisy background due to the preexisting modification layer before the detection, which may cause difficulties in interpreting the signal or poor signal-tonoise (S/N) ratios.201 More importantly, the specificity of modification layer based on electrostatic and hydrophobic interactions is not very high. Specific Molecular Receptors. To improve the specific affinity of a SERS substrate to an analyte, molecular receptors have also been employed to modify the substrate. Because of the small size of the hotspot, commonly used large molecular receptors in bio/chemical sensing, such as antigen/antibodies, biotin/avidin, aptamers, etc., are not well suited in this case. Therefore, small molecules are usually preferred as the specific molecular receptors.215,216 For example, SERS substrates modified with 4-mercaptophenylboronic acid are widely used for glucose sensing via reversible boronate ester formation.217−224 A triosmium carbonyl cluster-boronic acid conjugate functionalized SERS substrate was successfully used to detect glucose with concentrations as low as 10−4 M (Figure 7c).217 Compared with other methods or SERS substrate without modification, modified substrate can often achieve lower detection limits and simpler operation procedure (Table 1).225 Another typical example is using the proper molecules with both sensitive pH response and strong Raman signals for in vivo SERS-based pH sensing. The most commonly used probe molecules are 4-mercaptobenzoic acid226−228 and 4mercaptopyridine.229 Additionally, Ag nanoparticles functionalized with 2-mercaptoethanesulfonate can detect alkaline and alkaline earth metal cations.215 This method depends on the formation of contact-ion pairs between sulfonate groups of 2mercaptoethanesulfonate and metal cations. The LOD for Ca2+ was 10−8 M (Table 1). Thus, with the assistance of proper molecular receptors, molecules with weak affinity to SERS substrates or Raman-inactive ions could be detected. Mechanical Trapping. The mechanical trapping of analytes is achieved by stimuli-responsive SERS substrates. The most commonly used stimuli are pH,230,231 temperature,232−234 and light.235 For example, Au nanorod-doped poly(N-isopropylacrylamide) microgels were presented and applied for SERS application.234 It was demonstrated that the poly(N-isopropylacrylamide) microgels could collapse when heating the temperature beyond 32 °C; thus, the distance between the Au nanorods could be shortened correspondingly to generate the hotspots. When the temperature cooled down

extend a few nanometers (∼2 nm); accordingly, it is very difficult for analytes with a low affinity to enter such small, confined spaces. Therefore, surface modification of the SERS substrate to improve the contact between the analyte or to avoid competitive adsorption is essential for real-world SERS applications (Figure 7). A table summarizing the surface modification method is shown in Table 1. Electrostatic and Hydrophobic Interactions. Among the various approaches for improving the affinity of analytes to substrates, the simplest one is based on electrostatic and hydrophobic interactions. In most cases, the SERS colloids (i.e., citrate or borohydride reduced gold or silver particles) is negatively charged. As a consequence, to improve the affinity to most of organic molecules that often carry negative charges, the commonly used method is to modify the SERS substrates with a monolayer of positive-charged capping molecules (Figure 7a).16,191−196 For example, by functionalizing the amino group on the surface, negative-charged methylene blue can be selectively separated from the sample matrix containing acid blue and be detected.194 An alternative approach is to adjust the solution pH.197,198 By lowering the suspension pH below the analyte pKa, the surface affinity of SERS substrates can be improved because of the consistent electrostatic alignment. For example, using this technique, the detection limits (LOD) for atrazine in drinking water can be improved to 11 nM from 460 nM (Table 1).199 This method based on adjusting pH (without surface modification) overcomes the drawback of interfering background signal due to the modified molecules. However, uncontrolled aggregation of metallic colloid at low pH need to be aware. Furthermore, for analytes with no affinity for gold or silver or are highly hydrophobic, surface modification is commonly employed to attract target molecules to the SERS substrate via hydrophobic interactions, such as an alkanethiol self-assembled monolayer.200−204 On the basis of the strong Au−S bonds, a monolayer of alkanethiol can be easily assembled on the substrates, in order to entrap hydrophobic targets. Compared with unmodified SERS substrate, a lower LOD can often be achieved for these modified SERS substrates, as shown in Table 1.204 Another interesting surface modification strategy is based on the so-called host−guest interactions, in which a hydrophobic inner cavity and a hydrophilic outline to entrap the target molecules to the inner cavity.205−207 This method has gained special attention owing to its ability to capture a batch of small hydrophobic nonpolar molecules in a complex matrix (Figure 7b). For example, three PCB molecules captured by βcyclodextrin on the substrate surface is confirmed to be detected simultaneously,205 as shown in Table 1. The above approaches have been successfully applied to analyze real-world samples, such as blood plasma and serum for clinical diagnostics and contaminant tracing in food samples. As we know, competitive adsorption between analyte and other molecules, which results in an appreciable background noise, is a critical problem for real-world applications. To solve this problem, a smart strategy is to use a mixed self-assembled monolayer (SAMs) of thiol molecules and zwitterionic polymer, where the former is to attract or probe the target molecules and the latter is to repel nonspecific fouling from the complex sample matrix (Figure 7e).208,209 Moreover, a modified substrate with a surface composition of 94% N,Ndimethyl-cysteamine-carboxybetaine showed that low nonspecific fouling was obtained.210 Using this modified substrate, several analytes in undiluted plasma could be successfully 6532


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Figure 8. User-friendly SERS substrates: (a) in situ synthesized SERS substrates, (b) low cost flexible SERS substrates, (c) universal flexible substrates, and (d) integrated portable SERS devices. Copyright with the permission from refs 234 (Copyright 2015 Macmillan Publishers Ltd.), 245 (Copyright 2014 Wiley), 246 (Copyright 2013 Wiley), 252 (Copyright 2013 Elsevier), 253 (Copyright 2016 American Chemical Society), 254 (Copyright 2017 Elsevier), 256 (Copyright 2016 Elsevier), 257 (Copyright 2016 Wiley), 258 (Copyright 2015 Macmillan Publishers Ltd.), 266 (Copyright 2017 American Chemical Society), 267 (Copyright 2016 Elsevier), 274 (Copyright 2014 Royal Society of Chemistry), 281 (Copyright 2016 Elsevier), 285 (Copyright 2015 American Chemical Society), and 290 (Copyright 2014 American Chemical Society).

to 32 °C, the responsive matrix could recover to expanding status (Figure 7d). Therefore, analytes could be trapped in the hotspots by adjusting the temperature. Because of this, the sensitivity of this SERS substrate could be significantly improved compared to conventional SERS substrate (Table 1).236 In addition, dual stimuli-responsive microgels modified SERS substrates were also developed recently. For example, the hybrid microgels were confirmed to be utilized as for measuring both pH value and the temperature of their surroundings.237,238 Another alternative approach is molecularly imprinted polymerbased SERS detection, which usually consists of an ultrathin polymer layer imprinted with the targets.16,161,239,240 This method has been used in various fields, especially for biomolecules analysis (Table 1).241 This method was also successfully applied to selectively adsorb and separate αtocopherol from vegetable oils.242 Nevertheless, the enlarged distance between the analytes and surface of SERS substrates due to the polymer structure results in a modest SERS enhancement. 3.3. Strategies to Develop User-Friendly Real-Life SERS Applications. With the significant progress that has been made in solving the above fundamental problems such as poor signal reproducibility and low selectivity toward real samples, the SERS technique has thus reached the stage of developing real-life applications. In terms of practical applications, cost is a primary concern, and a good method should above all be an affordable one. However, most of the nanofabricated SERS substrates do not satisfy this prerequisite. Another concern is the convenience and easy usability of the application. For example, the above-mentioned SERS substrates, nanoparticle colloids, and rigid substrates can be conveniently used to analyze liquid forms of samples, but this is much more difficult in the quantification of a target molecule on roughed, irregular, and nonplanar surfaces, which is the most common case in real-life applications. Extra extraction processes are thus often required. In Situ Synthesized SERS Substrates. To meet the requirement of economic viability and operational simplicity, in situ synthesis of SERS substrates has been developed. For

example, combined with conductive ink, traditional pens (e.g., fountain pens and ball pens) can be employed to write conductive patterns on multiple substrates, such as glass243 or paper (Figure 8a).244,245 Using inkjet technology, microvolumes of metallic nanoparticle colloids have been directly sprayed onto the surface of artwork and textile fibers, which have been employed to in situ identify the organic colorants of Japanese woodblock print dating to the end of the 19th century (Figure 8a).246 It is worth mentioning that the former one, socalled “pen-on-paper” approach, can obtain a lower LOD of thiabendazole of 20 ppb.245 Similarly, this assay was also applied to quantify melamine in milk with a LOD of 0.27 mg/ L.247 Another in situ synthesis approach is to directly immobilize the plasmonic nanoparticles on the sample surface (e.g., bacteria cell wall20,248). The SERS signal obtained via this strategy is approximately 30-fold higher than in the case of a simply mixed colloid-analyte suspension and a lower LOD of 103 CFU/mL of L. innocua can be obtained.248 These methods are relatively low-cost, fast, and user-friendly, which have recently led to substantial research interest. However, the poor reproducibility of the Raman signal due to the irregularity of sample surfaces still remains a problem. Flexible SERS Substrates. A flexible SERS substrate refers to a SERS-active plasmonic nanostructure constructed on a flexible solid supporter. Such substrates exhibit several advantages over conventional rigid substrates in terms of easy usability and fabrication cost. Unlike the intrinsic rigid, fragile SERS substrates, these substrates can be attached to rough, irregular (i.e., nonplanar or ripply) surfaces and directly collect samples, offering a noninvasive or minimally invasive method of sample analysis. This advantage is especially useful for analyzing fragile and valuable samples. Examples of flexible SERS substrates include cellulose papers,249−252 polymers,253−255 graphene,256 etc. (Figure 8b). Our group has also developed a flexible patterned plasmonic metafilm by combining bottomup self-assembly and top-down laser engraving. With tunable plasmonic properties and excellent flexibility, this plasmonic material could be applied in various fields.257 Nevertheless, flexible SERS substrates can be fabricated via simple and cost6533


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Journal of Agricultural and Food Chemistry efficient approaches, such as the dip-coating method,250 printing technology,258 or interface self-assembly technique.259 An excellent review on flexible SERS substrates can be referred to for more details.260 The characteristics afforded by a flexible supporter enable flexible SERS substrates several new functionalities and extend to applications that were not possible before. For example, flexible SERS substrates can be easily cut into arbitrary shapes and integrated with other devices, such as for capillary-actuated fluid transport and selective molecular retention. Flexible SERS substrates can integrate the sample collection and the SERS detection into one, which is very convenient for in situ analysis.261,262 It offers other attractive advantages, such as being easy to transport or stable for storage. All of these advantages are expected to bring SERS technology closer to real-world applications. Currently, several groups have used the flexible SERS substrates to analyze chemicals in agricultural products and food. For example, via a simple “paste and peel off” approach, pesticide residues (e.g., parathion-methyl, thiram, and chlorpyrifos) in fruits and vegetables could be directly extracted to the as-prepared flexible SERS substrates and analyzed.254,263 The sensitivity of this approach can achieve a few nanogram per square centimeter (ng/cm2). Moreover, using a Ag decorated sandpaper substrate, the triazophos on fruit surface can be directly detected by swabbing the sample surface.264 The LOD can be achieved as low as 4.2 pM/cm2. Although flexible substrates can be easily attached on the irregular surface, limited by their poor optical transparency, additional sample exaction procedures (e.g., paste and peel off or swab254,263,264) are always needed for the detection. Therefore, in situ analysis of residues on the sample surface cannot be realized by these opaque SERS substrates. “All Tasks” SERS Substrates. The in situ quantification of chemical molecules on an ambient surface without any sample pretreatment has been a long-standing pursuit in analytical science. Among the few optional techniques, SERS is particularly attractive because of its ultrahigh sensitivity and selectivity. However, the most commonly used substrates, such as nanoparticle colloids or rigid substrates, cannot accomplish this task because nanoparticle colloids can hardly generate a reproducible signal over large areas and because rigid substrates cannot be attached to irregular surfaces. Additionally, conventional flexible substrates due to their poor optical transparency can completely block the signal after attaching to sample surfaces. Thus, the invention of flexible and transparent SERSactive substrates overcomes these dilemmas and can make the idea of in situ SERS detection a reality. Recently, various flexible substrates with transparent properties have been fabricated.259,265−271 There are two critical factors for the preparation of transparent flexible SERS substrates. Not only must the flexible supporter be transparent, the SERS-active metallic layer should also exhibit good optical transparency, which is difficult since the metallic layer is often not. A possible approach is using top-down method to fabricate thin metal nanostructures on a transparent polymer film but is limited by its high cost and low hotspot density.272 The commonly used method is to transfer an as-prepared nanoparticle monolayer array onto a polymer thin film. Our group also introduced a flexible and transparent SERS metafilm.273 A large-area 2D array of silver nanocubes (Ag NCs) was assembled via the LAI approach and then transferred onto cellophane adhesive tape. The obtained Ag NCs arrayattached tape exhibited excellent SERS activity and a

homogeneous enhancement factor; more importantly, its high mechanical flexibility and good transparency ensure its conformal contact with sample surfaces and enable excitation and collection of the signal from the backside of the substrate. Because of this feature, flexible substrates with transparent properties has become a promising method for in situ analysis of real-world samples. Recently, several groups have employed the flexible and transparent SERS substrate to direct analysis of chemicals in agricultural products and foods, such as pesticide residues and food additives (Figure 8c).259,266−268,274−277 By simply covering these substrates on the surface of samples, the contaminants can be directly detected. For example, pesticide residues can be in situ detected by attaching these substrates to the sample surfaces (e.g., fruit, fish). The sensitivity via this approach for detecting thiram on fruit peel and methylene-blue on fish surface can be down to 72 ng/cm2 and 10−13 M, respectively.266,271 Besides foods and agricultural products, residues on food packages such as malachite green on the inner wall of container can also be rapidly determined by transparent and flexible SERS substrates.273 Therefore, flexible and transparent SERS substrates could serve as a form of “all task” substrates since they suit all forms of samples, such as liquids, gases, solids, and especially analytes dispersed on an underlying nonplanar solid surface.273 It is far more suitable than other substrates for real-world applications, such as in homeland security, forensic science, environmental analysis, food safety, etc. Altogether, these features suggest the high versatility of flexible and transparent SERS-active substrates. Integrated Portable SERS Sensing Devices. To enable SERS technology to be applied to on field detection, portability is another important factor to be of concern. Most of the conventional techniques commonly require sophisticated sampling and separation procedures, which hamper their use in on field analysis. Recently, a variety of portable SERS sensing devices are developed, such as microfluidic chip or device,234,278−282 capillary or microcolumn,12,243,283,284 SERS enabled micropipet,285 paper-based SERS chip,261,286−288 SERS swab,289,290 lab-on-chip device,291 SERS test kit (Figure 8d).292 These on-site SERS platforms usually combine with portable Raman spectrometers which usually integrate separation, concentration, and quantification process all in one. For example, a SERS micropipet was developed for detecting pesticide residues (thiram, malachite green, and methyl parathion) on the vegetable surface. The LOD obtained via this device can be down to 8 nM, 8 nM, 1.5 μM for thiram, malachite green, and methyl parathion, respectively.285 Another SERS cotton swab was also fabricated for sensing surface residues. By simply swabbing the sample surfaces, the targeting contaminants could be collected and then detected. For example, 10−5 M carbaryl on a cucumber surface could be successfully discriminated.289 Thus, these portable and low-cost SERS-enabled devices make on-field application come true.

4. PERSPECTIVES Since its discovery, a tremendous amount of work has been invested in the field of SERS, which has successfully promoted our theoretical and experimental understanding of SERS. Especially with recent breakthroughs in nanotechnology, researchers have gained the ability to fabricate rationally designed and uniform plasmonic nanostructures as highperformance SERS substrates, extremely expanding the potential applications of SERS in various fields. However, 6534


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substrates to ensure enhancement of the absolute biomolecule signal.135,136 However, how to sensitively and selectively detect biomolecules using SERS still remains a challenge. Furthermore, to shorten the distance between laboratory research and real applications, portable Raman devices need to be considered. For example, a smartphone-based point-of-care genetic testing device that is inexpensive, user-friendly, and compact has been developed.302 A lot of mobile Raman spectrometers have also been developed in various on-site SERS applications.303 Combining rationally designed SERSactive substrates with portable Raman spectrometers, these SERS-enabled platforms, usually integrated separation, concentration, and quantification processes all in one, are worthy for future research.

most of the applications have, thus far, been limited to solving problems in the laboratory. To further expand the real-life applications of SERS, several challenges need to be addressed. This review attempted to emphasize these crucial but often overlooked problems and summarize possible solutions. We hope that our discussion will encourage more researches to address these challenges and eventually help to bring SERS technology out of the laboratory. The success of SERS application relies on the development of SERS-active substrates. Although SERS has been famous for its poor reproducibility, currently, reliable SERS applications can be developed based on highly uniform SERS substrates fabricated via top-down nanofabrication approaches, such as Ebeam lithography. The real reason for restraining SERS to the laboratory is the high fabrication cost, which is one of the primary concerns for developing real-life applications. Therefore, the further expansion of SERS depends on developing cost-efficient and high-performance substrates. Several possible strategies to address this challenge are listed above, either by improving conventional SERS colloids or by developing lowcost assembly techniques to fabricate uniform substrates. We can expect to see some practical SERS applications based on these advances in the near future. The design of SERS substrates used to focus on SERS-active nanostructures achieves a higher EF and better uniformity but what is often neglected is the affinity of the substrates to the analytes, which is a prerequisite for obtaining a reliable and sensitive SERS signal. Moreover, nonspecific fouling of a SERS substrate in a real sample matrix and weak affinity of the analytes to SERS-active substrates are the two main problems that need to be solved. Thus, how to combine a proper molecular capture strategy (such as preconcentration methods,293−295 specific recognition strategy296) with repelling fouling approaches to improve the sensitivity and selectivity of SERS detection is still an important issue for the future. A more delicate design approach should be carried out. One of the most promising and practicable application of SERS is the rapid testing for chemical residues in foods or agricultural products, such as pesticide. By simply spreading the SERS colloids on the sample surface or swabbing the sample surface with a soft SERS substrates, the residues could be rapidly analyzed.297−299 Besides, various user-friendly SERS sensing devices have been developed, such as microfluidic device,234 capillary,283,284 SERS micropipet,285 paper-based SERS test strip,287 and SERS swab,289 which were rationally designed for this purpose. The entire process, including preconcentration, separation, and SERS detection, can be completed within a few mintutes.297,300 Furthermore, recent development in transparent flexible SERS substrates can even allow analysis of chemical residues directly on the sample surface by directly attaching this substrate to the sample surface and recording SERS signal from its back.301 These advances will definitely benefit to bring SERS technology closer to real-world applications. Further Challenges. The mismatch between the size of the hotspot and that of the target is one of the main problems for SERS detection of biomolecules. Thus, we can obtain only a partial SERS signal of biomolecules; therefore, a fluctuation in the SERS signal is expected due to intrinsic distance-dependent effects of SERS and the relatively large and complex structures of the biomolecules. Several approaches have been employed to overcome this problem, such as surface modification, which aimed to slightly enlarge the distance between analytes and

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Xiangjiang Liu: 0000-0002-5419-669X Yibin Ying: 0000-0002-3392-9380 Funding

This work was financially supported by the National Natural Science Foundation of China (Grant No. 31571922), the Natural Sciences Fund of Zhejiang Province (Grant No. LY14B050004). Notes

The authors declare no competing financial interest.

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REFERENCES

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