Food Quality Monitor: Paper-Based Plasmonic Sensors Prepared

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Food Quality Monitor: Paper-Based Plasmonic Sensors Prepared Through Reversal Nanoimprinting for Rapid Detection of Biogenic Amine Odorants Shih-Yu Tseng, Szu-Ying Li, Shang-Yi Yi, Aileen Y. Sun, Dong-Yu Gao, and Dehui Wan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 04 May 2017 Downloaded from http://pubs.acs.org on May 4, 2017

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ACS Applied Materials & Interfaces

Food Quality Monitor: Paper-Based Plasmonic Sensors Prepared Through Reversal Nanoimprinting for Rapid Detection of Biogenic Amine Odorants

Shih-Yu Tseng, Szu-Ying Li, Shang-Yi Yi, Aileen Y. Sun, Dong-Yu Gao, and Dehui Wan*

Institute of Biomedical Engineering, National Tsing Hua University, Hsinchu, Taiwan

*

To whom correspondence should be addressed. E-mail: [email protected]

Keywords: Localized Surface Plasmon Resonance, Nanoimprinting, Paper-based Sensors, Biogenic Amines, Food Safety

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Abstract This paper describes the fabrication of paper-based plasmonic refractometric sensors through the embedding of metal nanoparticles (NPs) onto flexible papers using reversal nanoimprint lithography. The NP-embedded papers can serve as gas sensors for the detection of volatile biogenic amines (BAs) released from spoiled food. Commercial inkjet papers were employed as sensor substrates—their high reflectance (>80%) and smooth surfaces (roughness: ca. 4.9 nm) providing significant optical signals for reflection-mode plasmonic refractometric sensing and high particle transfer efficiency, respectively; in addition, because inkjet papers have light weight and are burnable and flexible, they are especially suitable for developing portable, disposable, cost-effective, eco-friendly sensing platforms. Solid silver NPs (SNPs), solid gold NPs (GNPs), and hollow Au–Ag alloyed NPs (HGNs) were immobilized on a solid mold and then transferred directly onto the softened paper surfaces. The particle number density and exposure height of the embedded NPs were dependent on two imprinting parameters: applied pressure and temperature. The optimal samples exhibited high particle transfer efficiency (ca. 85%), a sufficient exposure surface area (ca. 50% of particle surface area) presented to the target molecules, and a strong resonance reflectance dip for detection. Moreover, the HGN-embedded paper displayed a significant wavelength dip shift upon the spontaneous adsorption of BA vapors (e.g., ∆λ = 33 nm for putrescine; ∆λ = 24 nm for spermidine), indicating high refractometric sensitivity; in contrast, no visible spectroscopic responses were observed with respect to other possibly co-existing gases (e.g., air, N2, CO2, water vapor) during the food storage process, indicating high selectivity. Finally, the plasmonic sensing papers were used to monitor the freshness of a food product (salmon).

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Introduction Metal nanoparticles (NPs) have been widely exploited for novel applications in environmental monitoring,1 biomedical diagnosis,2 and food safety.3 In particular, metal NPs are attractive for their localized surface plasmon resonance (LSPR) properties.4 The LSPR peaks of metal NPs show significant spectroscopic responses to variations in their compositions, morphologies and of the refractive index (RI) of the surrounding medium.5-9 Therefore, the past 15 years have seen an explosion of research in the field of LSPR-based chemical and biological sensors,10-19 including colorimetric sensing based on particle–particle coupling10 or morphology/composition evolution,14,

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refractometric sensing,16,

17

Rayleigh scattering–based waveguide

sensing,18 and surface-enhanced Raman scattering (SERS).19 Herein, we demonstrate that plasmonic metal NPs can be employed for an important niche application: food freshness indication, where the properties of the NPs are directly related to their efficacy and performance. Refractometric sensing is a potentially powerful method for NP-based sensing because this approach is very sensitive to changes in RI close to the NP surface.12, 16 Such a change may occur upon analyte adsorption on the NP surface, either directly or through a pre-modified layer, resulting in a wavelength red-shift and an extinction increase of the LSPR peak.20 Accordingly, previous studies have focused primarily on the detection of bulky biomolecules (e.g., DNA, proteins)21, 22 that could result in a large change in the surrounding RI. In contrast, LSPR sensing for gas detection has been challenging because the adsorbed gas molecules generally induce minute RI changes. Recently, an increasing number of studies have been conducted on small gas molecules (e.g., H2,23, 24 CO2,25 NO2,26 volatile organic compounds27) by developing metal NPs modified with functional layers to facilitate preferential adsorption of gaseous analytes. For example, Chen et al. constructed a sensing platform based on 3



ferrocene-functionalized Au to sensitively detect NO2, taking advantage of active molecular plasmonics.26 Similarly, Li et al. employed the high adsorption ability of Pd towards H2 to significantly enhance the sensitivity of Au-based H2 sensors.24 Although these LSPR-based gas sensors displayed high sensitivity, the fabrication of such functionalized nanostructures generally requires sophisticated manufacturing processes, potentially restricting the gaseous analytes capable of detection. The majority of research related to refractometric sensing has been performed using metal NPs immobilized on solid substrates,16, 17, 22-28 rather than solution-based NPs.29 Immobilization of NPs prevents unwanted aggregation, which may occur during processing or storage in solution. Furthermore, immobilized metal NP arrays provide a convenient platform for detection of various analytes, in either the liquid or gaseous phase. Glass is the most popular material employed as a solid substrate for plasmonic sensors, due to its high transparency and the availability of well-established NP immobilization methods.17 Glass slides are, however, generally rigid, fragile, heavy, and relatively difficult to fabricate and miniaturize based on current manufacturing technologies,30 limiting their practical sensing applications, especially in point-of-care diagnosis, on-site environmental monitoring, and food safety controls. Paper has recently been rediscovered as a supporting material for low-cost sensing platforms for diagnostic applications,30-32 because paper substrates provide thinness, flexibility, light weight, and abundant storage capability.30 Furthermore, metal NPs have been employed to improve the sensitivity, stability, and cost-efficiency of paper-based sensors.33 Therefore, several strategies have been developed to fabricate NP-containing papers.34-43 One common method is the immersion of a paper substrate into a colloidal solution, followed by ambient drying.34, 35

For instance, Lee et al. employed Au nanorods to prepare a paper-based SERS

substrate using the immersion approach.34 The chemically synthesized Au nanorods 4


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adsorbed and then aggregated on the paper’s fibers, providing a large number of hot spots to enhance the Raman signals of the analytes. Nevertheless, this method requires very long immersion times (typically 48 h), with aggregation of the NPs decreasing the surface area of the NPs exposed toward the incoming analytes, thereby decreasing the refractometric sensitivity of the NPs. Inkjet printing is an alternative method for preparing metal NP arrays on paper substrates.36,

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For example, Creran et al.

synthesized enzyme-coated Au NPs in a colloidal solution, and sequentially printed the NPs onto papers with an inkjet printer.37 There are, however, disadvantages: tedious routines for optimizing the NP ink formulation and expensive equipment. Moreover, metal NPs have been formed in situ on papers by immersing the papers into precursor solutions, together with reducing agents38 or ultrasonic radiation.39 It is, however, difficult to obtain complex-shaped metal NPs (e.g., triangles, rods, cages) when using this method. Such metal nanostructures generally exhibit longer LSPR wavelengths, resulting in higher sensitivity to surrounding RI changes, when compared with the behavior of metal spheres.29, 44 Furthermore, physical formation methods have recently been developed for fabricating NP-containing paper substrates, due to the poor wettability of paper-based substrates.40-42 For example, Jung et al. developed a low-temperature annealing method to prepare metal NP–containing papers.42 Metal thin films were first deposited onto a paper substrate through evaporation, and then metal NP arrays were produced through thermally annealing. This physical formation allows large-area throughput and the capability of preparing NP arrays in high density on paper substrates. Nevertheless, the high cost of the deposition equipment decreases the practicality of the large-scale production of NP-based sensors. More importantly, this method also lacks the shape-tunability of metal NPs, potentially resulting in poor refractometric sensitivity. Noteworthily, the current strategies reported for forming—both chemically and physically—metal NP 5



arrays

on

papers

are

either

complicated/expensive

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or

cause

poor

spatial-dispersion/shape-tunability of the NPs; thus, there is still room for improvement. In this study, we fabricated paper-based plasmonic refractometric sensors through the embedding of metal NPs onto flexible paper substrates using reversal nanoimprint lithography (rNIL).45 This fabrication approach combines the attractive features of both chemical and physical formation: (i) the metal NPs were chemically synthesized in a colloidal solution, thereby providing flexible tunability of the NP composition/shape;46 (ii) the metal NPs were then immobilized onto reusable solid molds (e.g., glass, silicon) through a facile NP self-assembly process,17 thereby routinely producing monolayer, well-dispersed NP arrays in high density; (iii) the metal NP arrays were transferred directly from solid molds onto paper substrates without any surface-modification, via rNIL technology, thereby providing robust and rapid fabrication (requiring only a few minutes), large-area throughput, and cost-effective processing.47 Therefore, we believe that the NP-embedded papers can be developed into simple, inexpensive, portable, disposable, and highly sensitive LSPR-based refractometric sensors. As a proof of the concept, we employed an as-fabricated NP-embedded paper to serve as a gas sensor for the detection of volatile biogenic amines (BAs), which are strongly related to the freshness of a variety of food products, especially fish and meat.48-55

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Results and Discussion

Figure 1 Schematic representations of (a) the imprinting of metal NPs onto an inkjet paper and (b) NP-embedded paper serving as a gas sensor to detect biogenic amine odorants released from spoiled food.

Figure 1 illustrates the concepts of (i) using rNIL process to prepare a NP-embedded sensing paper, and (ii) applying the NP-containing paper for the detection of BA odorants released from spoiled food. First, we immobilized monolayer metal NPs onto rigid molds pre-modified with amino groups. Notably, the metal NPs exhibiting different compositions and morphologies could be pre-synthesized chemically or obtained commercially. In our case, the solid silver NPs (SNPs) were synthesized through the polyol-reduction method, and the hollow Au–Ag alloyed NPs (HGNs) were synthesized through the galvanic replacement reaction.29 The average diameters of the SNPs and HGNs were 56 and 66 nm, respectively. The solid gold NPs (GNPs) having an average diameter of 50 nm were obtained commercially. Figures 2a–c reveal the TEM morphologies of the SNPs, GNPs, and HGNs, respectively; Figure 2d shows the extinction spectra recorded from their colloidal solutions. The SNPs exhibited a typical LSPR peak near 410 nm; the GNPs exhibited an LSPR peak near 7



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530 nm; while the HGNs exhibited an apparently red-shifted LSPR peak near 810 nm because of their thin shells and hollow morphologies29 (see Figure 2c). The metal NPs were subsequently transferred directly onto paper substrates from the molds using rNIL (see Figure 1a). Imprinting was performed under a variety of applied pressures (20–40 kg/cm2) and at various temperatures (25–120 °C). Under these imprinting conditions, the surfaces of papers can be softened, resulting in the embedding of the NPs into the soft surface. After demolding, the papers were successfully functionalized with a monolayer array of metal NPs. The transfer of these NPs could be explained by that the adhesion force (primarily van der Waals forces) between the embedded NPs and the paper was greater than that between the immobilized NPs and the mold. To demonstrate the refractometric sensitivity of the LSPR-based paper sensors, we chose volatile BAs as gaseous analytes and observed the spectroscopic responses of NP-embedded papers toward the adsorption of BAs (Figure 1b). BAs are organic bases of low molecular weight and high volatility.54, 55 The main BAs found in spoiled food products are putrescine (PT), spermidine (SP), histamine, cadaverine, tyramine, and spermine,50-52 which typically have one or more terminal amino groups. During the detection process, the gaseous BAs would attach spontaneously onto the surface of the NPs via their amino groups,56 forming a dense layer around the NPs stabilized through intermolecular interactions.57,

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The adsorption of BAs should,

therefore, induce a change in the local RI around the NPs, resulting in a red-shift of the LSPR wavelength. Notably, BAs are generated in foods through microbial decarboxylation of amino acids mediated by specific bacterial strains, especially under inappropriate storage and handling processes.49-53 In other words, the levels of BAs can be an indicator of freshness and quality of food,48,

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because the BAs

accumulate in foods along with the growth of microorganisms, which may lead to foodborne poisoning. Therefore, in this study, we anticipated that our plasmonic 8


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papers might serve as food freshness indicators operating through the detection of BA odorants released from spoiled food (salmon).

Figure 2 TEM images of (a) SNPs, (b) GNPs, and (c) HGNs (average sizes: 56, 50, and 66 nm, respectively). (d) UV–Vis spectra of aqueous suspensions of SNPs, GNPs, and HGNs. Scale bar = 50 nm.

To choose a suitable paper substrate for developing LSPR-based sensors fabricated through imprinting, we examined the optical properties and surface roughnesses of six types of commonly used paper: cellophane, nonwoven fabric, waxed paper, tracing paper, filter paper, and inkjet paper. The filter paper and the inkjet paper displayed high reflectance (>75%) over the spectral range 400–1000 nm, whereas the others had much lower reflectance (

Food Quality Monitor: Paper-Based Plasmonic Sensors Prepared

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