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Binary Thiol-Capped Gold Nanoparticle Monolayer Films for Quantitative SERS Analysis Huihui Tian, Hongbian Li, and Ying Fang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b02069 • Publication Date (Web): 08 Apr 2019 Downloaded from http://pubs.acs.org on April 10, 2019

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ACS Applied Materials & Interfaces

Binary Thiol-Capped Gold Nanoparticle Monolayer Films for Quantitative SERS Analysis Huihui Tian*, †, ‡, Hongbian Li †, ‡, Ying Fang*, †, ‡, § †

CAS Key Laboratory for Biomedical Effects of Nanomaterials and Nanosafety, CAS Center

for Excellence in Nanoscience, National Center for Nanoscience and Technology, Beijing 100190, China ‡

University of Chinese Academy of Sciences, Beijing 100049, China

§

CAS Center for Excellence in Brain Science and Intelligence Technology, 320 Yue Yang

Road, Shanghai 200031, China

ABSTRACT: Surface Enhanced Raman Scattering (SERS) can provide fingerprint information of analyte molecules with unparalleled sensitivity. However, quantitative analysis using SERS has remained one of the major challenges owing to the difficulty of obtaining reproducible SERS substrates with high-density hotspots. Here, we report the rational design and fabrication of a binary thiol-capped gold nanoparticle (AuNP) monolayer film (MLF) as substrate for highly sensitive and quantitative SERS analysis. The two thiol ligands chemically bonded to the AuNPs play different roles: the dodecanethiol (DDT) with long alkyl chain controls the interparticle gaps and electromagnetic coupling among AuNPs, and the 4-mercaptopyridine (Mpy) works as Raman internal standard (IS). The binary thiol-capped AuNPs can self-assemble into an ordered MLF with high-density hotspots and uniformly distributed IS. The as-prepared MLF has been demonstrated as reliable SERS 1

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substrate for quantitative detection of fungicide malachite green (MG) in aqueous solution, with a high enhancement factor (EF) (up to 3.3 × 107) and a low detection limit (100 pM). Moreover, the MLF SERS substrate is flexible and transparent, which has enabled in-situ detection of trace fungicide residues in shrimp tissue.

KEYWORDS: Quantitative SERS; self-assembly; gold nanoparticle; monolayer film; malachite green

1. INTRODUCTION

SERS is a powerful analytic technique for the detection and identification of trace chemical and biological molecules.1-9 In SERS, molecular specific Raman signals are enhanced by placing the analyte molecules in close proximity to a plasmonic nanostructure. In particular, nanoscale junctions between metal nanoparticles, known as “hotspots”, can dramatically enhance local electromagnetic (EM) fields and thus allow ultrasensitive analysis of molecules located inside the hotspots.10 However, owing to the near-field nature of SERS phenomenon, the EFs in the hotspots are highly sensitive to the size, shape, and spacing of the nanoparticles,11-18 as well as local dielectric environments.19-21 As a result, quantitative SERS analysis has been a major challenge in practical applications because of the difficulty in obtaining uniform and reproducible SERS substrates.1,22 Recently, self-assembled nanoparticle monolayers with high-density and uniform hotspots have attracted great interests as SERS substrates.15,23-27 In particular, self-assembled AuNP monolayers offer the advantages of high plasmonic activity in the visible to near infrared regions,10 chemical stability,3 well-tunable nanostructures,15,17 as well as the simplicity and scalability of their 2


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preparation.28-34 However, the EFs of self-assembled AuNP monolayers can still vary by orders of magnitude due to sample and instrument variation, such as laser power, integration time, and polarization of incident light.22

In order to improve the reproducibility of quantitative SERS analysis, an internal standard (IS) molecule can be introduced to the analyte sample to correct the instrumental variation.22,35,36 However, a competitive adsorption on SERS substrate can occur between the analyte and the IS molecules. In addition, the SERS signals of the IS molecules are sensitively dependent on the surrounding microenvironment.3,22,37 To address these issues, core-molecule-shell nanoparticles (CMS NPs) have been synthesized, in which the IS molecules was sandwiched between the core and shell of noble metal nanoparticles.38-41 CMS NPs can provide a stable microenvironment for the IS molecules. However, the preparation process of the CMS NPs is relatively complex. Recently, self-assembled silver nanoparticles (AgNPs) with a compact alkylthiolate monolayer have been demonstrated as reliable SERS substrates for quantitative analysis.24 The alkylthiolate molecules were chemically bonded to the AgNP surface and used as both the stabilizing agent to control the interparticle distances and the internal SERS reference for quantitative analysis. However, alkylthiolate molecules have weak SERS signals, which greatly limited their application as IS molecules in highly sensitive SERS analysis.

Here, we design and fabricate a highly sensitive and reliable SERS substrate based on large-area, self-assembled MLF of AuNPs capped with two thiolate ligands, Mpy and DDT. Both Mpy and DDT can stably bind to the AuNPs through Au-S bonds. The Mpy ligand has 3



strong SERS response and works as the IS. The DDT ligand works as the competitor for binding sites on AuNPs, which controls the surface density of the Mpy and thus the SERS intensity of the IS. The long DDT ligand also works as the spacer to control the interparticle distance between AuNPs in the MLF. The close-packed superlattice of the AuNP MLF provides high-density hotspots that are homogenously distributed over large areas. The as-prepared binary thiol-capped AuNP MLF has been demonstrated as sensitive and reliable SERS substrate for quantitative detection of fungicide malachite green in aqueous solution. Moreover, the SERS substrate is highly flexible and transparent, which has been further applied for in-situ detection of fungicide residues in shrimp tissue.

2.

EXPERIMENT SECTION

2.1 Chemical Reagents Hydrogen tetrachloroaurate (III) trihydrate (HAuCl4·3H2O, 99.9%), sodium citrate (C6H5Na3O7, 99%), citric acid (C6H8O7, 99%), and ascorbic acid (C6H8O6, 99%) were obtained from Alfa Aesar. Acetone (99%, AR) and hexane (99%) were purchased from Sinopharm Chemical Reagent Co. (China). Poly(methyl methacrylate) (PMMA, 8%) was from MicroChem. Mercaptopyridine (Mpy, 99%) was from Shanghai Aladdin Bio-Chem Technology Co., LTD. Crystal violet (C25H30ClN3, > 99%), Malachite green (MG, C23H25N2·Cl, > 99%), and Dodecyl mercaptan (DDT, > 99.5%) were purchased from Sigma Aldrich Co., LLC. All reagents were of analytical grade and used without further purification.

2.2 Synthesis of AuNPs The AuNPs of different diameters were prepared with a seed growth method according 4


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to previous report, with ascorbic acid as the reductant and sodium citrate as the stabilizer.42,43 Seed particles of ~ 15 nm were firstly prepared by the standard citrate reduction method.43 In brief, 2 mL reducing solution containing sodium citrate (1% w.t.) and citric acid (0.05% w.t.) was quickly added into 50 mL of boiled HAuCl4 solution (0.01%) under vigorous stirring. After 5 min, the reaction was stopped and the mixture was cooled down to room temperature. For the synthesis of AuNPs of different diameters (Figure S1), a certain volume of seed solution was firstly diluted to 20 mL and added into a three-necked flask. Then 10 mL diluted HAuCl4 stock solution and 10 mL diluted reducing solution containing ascorbic acid and tri-sodium citrate stock solution were added separately through Teflon tubes via a peristaltic pump at the same speed of 0.25 mL/min under vigorous stirring. The detailed added volume of the seed solution (V1), HAuCl4 stock solution (V2, 0.2% w.t.), and ascorbic acid (V3, 1% w.t.) and tri-sodium citrate (V4, 1% w.t.) stock solution can be found in Table S1.

2.3 Preparation of thiol-capped AuNP MLF SERS substrates Taking the preparation of Mpy/DDT-15 nm-AuNP MLF as an example (Figure S2), 10 mL of acetone was firstly added into 10 mL of aqueous Au colloid and shaken for 1 second. Then 10 mL of hexane containing 1 μM Mpy and 49 μM DDT was promptly added to the solution, followed by a vigorous shaking process. After 1 min, the mixed solution was transferred to a petri dish quickly. The AuNPs at the water/hexane interface shrank gradually under surface tension and finally assembled into a close-packed film in 10 min. Then most of the hexane on the top layer was removed by a syringe, and the left hexane was washed with pure hexane for three times to remove excess free thiolate molecules. After the complete vaporization of the remaining hexane, self-assembled MLF of binary thiol-capped AuNPs 5



was obtained. A thin PMMA film was firstly spin-coated on an Al foil at a speed of 200 rpm. After baking at 120℃ for 12 min, the PMMA/Al was immersed in water and then raised slowly out of water to scoop up the AuNP film floating at the air−liquid interface. The sample was then dried at 120℃ for 30 min to remove the residual water completely. Then the sample was immersed in ethanol for 1 min to wash off any adsorbed DDT and Mpy molecules, followed by washing with distilled water for 3 times and drying under room temperature. The Al foil was etched off in 1.0 M FeCl3 solution, and a transparent, flexible and free-standing Mpy/DDT-AuNP MLF SERS substrate was obtained. The preparation of DDT-AuNP MLF was the same with that of Mpy/DDT-AuNP MLF, except that only 50 μM DDT in hexane was used.

2.4 SERS measurement To investigate the surface uniformity of the substrates, a transparent MPy/DDT-AuNP MLF SERS substrate was immersed in 2 × 10-9 M MG solution for 1 h. The adsorption saturation time of the analyte on the MLF was determined to be ca. 30-40 min (Figure S3). Then the sample was taken out and washed with distilled water for three times. The sample was put on a glass slide for Raman measurements with AuNP array side facing up or down, which are termed as top incident mode and back incident mode, respectively. The in-situ SERS measurement of crystal violet (CV) aqueous solution or MG in solution was carried out with back incident mode. The as-prepared DDT-AuNP MLFs or Mpy/DDT-AuNP MLF/PMMA was supported by a windowed scotch tape to form the SERS substrates. The SERS substrate was put on the surface of the analyte solution for 1 h, with AuNP array side facing to the solution. Then the sample was irradiated with laser for Raman characterization. 6


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To detect the MG residue in shrimp, the shrimp was firstly fed in MG solution of different concentrations for 1 h. The shrimp was washed with water thoroughly for three times to remove the adsorbed MG on the shell. Then a 1 × 1 cm2 shell together with the protection layer were removed before the Mpy/DDT-AuNP MLF SERS substrate was pasted onto the surface of the shrimp, with AuNP array side in contact with the shrimp tissue. The sample was stored in a humid box for 1 h before SERS characterization.

2.5 Characterization The morphology of DDT-AuNP MLF and Mpy/DDT-AuNP MLF was characterized with scanning electron microscope (SEM) (Hitachi S8220) and transmission electron microscope (TEM) (Tecnai G2 20 S-TWIN, FEI). Optical characterization was carried out by a UV-vis spectrophotometer (LAMBDA650, PerkinElmer). The SERS spectra were collected on a Raman spectrometer (Renishaw inVia plus, England). A He-Ne 632.8 nm laser line was used for the measurements, and the laser spot was approximately 2 μm in diameter.

3. RESUTLD AND DISCUSSION

Figure 1a illustrates the self-assembly and transfer process of the AuNP MLFs. An aqueous colloidal solution of AuNPs with uniform size and shape was prepared by a seed growth method with sodium citrate as a reducing and surface-stabilizing agent.42,43 The citrate anion-stabilized AuNPs were negative charged and hydrophilic. Acetone was added and quickly mixed with the aqueous solution to reduce the surface charges on the AuNPs.44 Then a hexane solution containing Mpy and DDT was added, followed by vigorous shaking, to facilitate the replacement of the citrate anions on the AuNPs with the thiolates. The red-wine 7



colloidal solution quickly turned colorless as the AuNPs moved toward the hexane/water interface (Figure S2). The interfacial tension at the hexane/water interface compressed the AuNPs into a close-packed and ordered MLF. Figure 1b shows a ca. 40 cm2 binary thiol-capped AuNP MLF floating on water surface after the removal of the hexane. The as-prepared MLF can be readily transferred onto arbitrary substrates, such as PMMA and quartz films (Figure 1b and 1c). In particular, we used 1 μm-thick PMMA films as the support substrate for AuNP MLFs due to their high flexibility and transparency, as well as minimal Raman background (Figure S4). As shown in Figure 1b, a binary thiol-capped AuNP MLF on PMMA substrate can be conformally attached onto a 2 mm-diameter capillary tube. TEM image of an AuNP MLF in Figure 1d reveals that the AuNPs assembled into a hexagonal-close-packed (hcp) superlattice with an interparticle spacing of ~ 2-3 nm. An expected red-shift of the highly ordered MLF compared to AuNP colloid may be clearly observed from the corresponding UV-vis spectra (Figure S5).

Figure 1. Preparation of binary thiol-capped AuNP MLFs. (a) Schematic of the fabrication 8


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process of binary thiol-capped AuNP MLFs; (b) Photograph of the side view (left column, scale bar is 1 cm) and front view (right top, scale bar is 0.5 cm) of a large-area AuNP MLF floating on the surface of water, and a flexible AuNP MLF on PMMA substrate attached onto a 2 mm-diameter capillary tube (right bottom, scale bar is 2 mm); (c) Photographs of a AuNP MLF on quartz substrate with blue transmitting color and goldish reflection color (scale bars are 1 cm); (d) TEM image of a AuNP MLF with hexagonal superlattice (scale bar is 100 nm). We systematically evaluated the size-dependent properties of the assembled AuNP MLFs. Figures 2a-c are TEM images showing close-packed MLFs of DDT-capped AuNPs with diameter of 15 nm, 45 nm, and 78 nm, respectively. The interparticle gaps in the three MLFs are determined by the DDT chain length and in the range of 2±1 nm. Note that this interparticle distance has been shown to be optimal for strong near-field EM coupling.19,28,45 SEM images of the MLFs further confirmed the homogeneous and dense packing of the AuNPs over large areas (Figure S6 and S7). The SERS performance of the assembled DDT-AuNP MLFs was evaluated by using crystal violet (CV) as a model Raman analyte.19,27,32 The SERS substrates are denoted as MLF-15, MLF-25, MLF-45, MLF-78 and MLF-95 for the MLFs with AuNPs of 15 nm, 25 nm, 45 nm, 78 nm, and 95 nm in diameter, respectively. As shown in Figure 2d, all MLF SERS substrates provide strong Raman enhancement effects on CV with concentration of 10 nM. The EFs of the SERS substrates were calculated using the intensity of the in-plane mode peak at 1621 cm−1 of CV. Taking the MLF of AuNPs with diameter of 45 nm as an example (Figure S8), the EF of the SERS substrates were calculated using the following equation:46 ⁄

(1)

⁄

9



where ISERS and Ibulk are the measured SERS intensity for CV on the AuNP MLF substrate, and the measured normal Raman scattering (NRS) intensity of CV bulk solution; Nsurf and Nbulk represent the number of the molecules that corresponding to the surface and solution that effectively excited by a laser beam; C is the concentration of adsorbate in solution; NA is the Avogadro constant; A is the area of the focused laser spot; σ is the surface area occupied by each molecule, which is calculated to be 1.95 nm2;47 h is the effective laser waist, which is ~ 60 µm;46 R is the roughness factor of an electrode, which is estimated to be 1.73, assuming that only half of the AuNPs are exposed to the solution. The EFs for MLF-15, MLF-25, MLF-45, MLF-78, and MLF-95 were determined to be 8.2 × 106, 2.0 × 107, 3.3 × 107, 1.2 × 107, and 2.7 × 106, respectively. The high EFs of our AuNP MLFs can be attributed to the high-density hotspots generated in the nanoscale junctions of the densely-packed AuNPs.19 The MLF-45 showed the highest EF of 3.3 × 107 and was thus chosen for further evaluation. The measured EF of a nanoparticle array is determined by both the electromagnetic (EM) field intensity and the total available field volume.19,27,32,45 Former studies have shown that the maximum field intensity in a nanoparticle gap increases with increased diameter-spacing ratio, whereas the total available field volume decreases with increased diameter-spacing ratio.32,45 In our work, the interparticle spacings in the MLFs are determined by the DDT chain length and in the range of 2±1 nm for all the arrays. Therefore, the diameter-spacing ratio is linearly related to the nanoparticle diameter. As a result, the EM enhancement factor of an array increases first and then decreases with increased nanoparticle diameter. Here, MLF-45 shows the highest EM enhancement. Similar results have been reported previously.27,45 10


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Figure 2. Size-dependent Raman enhancement of DDT-AuNP MLFs. (a-c) TEM images of MLF-15, MLF-45 and MLF-78. The scale bars in (a-c) are all 100 nm; (d) SERS spectra of 10-8 M CV on MLFs with AuNP diameters from 15 nm to 95 nm. Next, we investigated the uniformity of the binary thiol-capped AuNP MLFs in Raman enhancement. Figure 3a illustrates a representative Raman spectrum of a Mpy/DDT-45 nm-AuNP MLF, with Mpy:DDT = 1:49, in which the fingerprint Raman peaks of Mpy can be clearly identified. Figure 3b shows the Raman spectra collected at 64 different spots in a 16 × 16 µm2 region. Notably, the Raman spectra are highly reproducible across the whole region. In addition, Figure 3c presents the Raman mapping of the Mpy peak at 1090 cm-1, which yielded a low standard deviation of only 4.5% across the 16 × 16 µm2 region. These results confirm that Mpy molecules are homogenously distributed over the assembled AuNP MLF and can be used as IS for quantitative SERS analysis.

11



Figure 3. Uniformity of Mpy/DDT-45 nm-AuNP MLF SERS substrate. (a) Representative Raman spectra of Mpy IS; (b) Raman spectra of Mpy measured at 64 spots in a 16 × 16 μm2 region; (c) Two-dimensional Raman mapping image of Mpy peak at 1090 cm-1 for the spectra in (b); (d) Comparison of Raman spectra of 2 × 10-9 M MG solution collected with top (red) and back incident mode (black), respectively; (e) Raman spectra of MG collected from 18 spots that randomly distributed in a region of 15 × 15 μm2. The peaks marked by the asterisk are the 1090 cm-1 band (red) of Mpy and 1172 cm-1 band (blue) of MG molecule, respectively; (f) Normalized SERS intensity (IR) of MG peak at 1172 cm-1 to that of Mpy at 1090 cm-1 for Raman spectra shown in (e). Laser power was 2 mW, and the integration time was 10 s. The high EFs and uniformity of the binary thiolate-capped AuNP MLFs make them an attractive substrate candidate for quantitative Raman analysis. Malachite green (MG) is an illegal antimicrobial in aquaculture that has been restricted or banned in many countries because of its carcinogenesis, teratogenicity, and mutagenicity.36 However, due to its low cost and high efficacy, MG is still being used in many parts of the world. Next, we applied the Mpy/DDT-45 nm-AuNP MLF SERS substrates for quantitative detection of MG in aqueous solutions. Owing to the high transparency of the PMMA substrate, the SERS spectra of MG 12


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collected from the top and bottom sides of the substrate are almost identical (Figure 3d). Therefore, Raman measurements can be carried out with both top and back incident mode. Figure 3e shows the Raman spectra of MG collected in top incident mode from 18 spots randomly distributed in a 15 × 15 μm2 region. We calculated the intensity ratio (IR) of the MG spectral band at 1172 cm-1 (ring C-H in-plane vibration) to that of the Mpy band at 1090 cm-1 (typical ring breathing/C-S of pyridyl). As shown in Figure 3f, the standard deviation of IR over the 15 × 15 μm2 region is only 6.3%, indicating that the MG molecules are evenly absorbed over the SERS substrate.

Figure 4a shows representative Raman spectra of MG at different concentrations from 10-10 to 10-7 M. It was noticed that the intensity of the Mpy peak at 1090 cm-1 was kept relatively constant, indicating that there was minimal competitive adsorption between the IS and the analyte even at high analyte concentrations. These results indicate that DDT is highly effective in protecting Mpy from analyte molecules. We note that all the Raman bands of MG are clearly visible even at a concentration as low as 10-10 M (Figure 4b and Figure S9), showing that the SERS substrate is highly active in the Raman signal enhancement of the MG molecules. Figure 4c summarizes the Raman spectra of MG at low concentrations from 10-10 to 10-8 M. The Raman intensity of the MG bands increased sensitively with the increase of MG concentrations. We normalized the intensity of the MG band at 1172 cm-1 to the Mpy band at 1090 cm-1. As shown in Figure 4d, the working curve can be well fitted to the Langmuir isotherm model with R2 = 0.995 from 10-10 M to 10-6 M. Therefore, the MLF can serve as a reliable and sensitive substrate for quantitative SERS analysis over a large concentration range. 13



Figure 4. Quantitative analysis of MG with Mpy/DDT-45 nm-AuNP MLF SERS substrate. (a) Raman spectra of MG with different concentrations from 10-10 to 10-7 M. Inset is an image of real-time measurement of MG in aqueous solution with Mpy/DDT-45 nm-AuNP MLF SERS substrate; (b) Raman spectrum of 10-10 M MG; (c) SERS spectra of MG at a concentration ranging from 10-10 to 10-8 M. Red star highlights the Mpy Raman peak at 1090 cm-1, and blue star highlights the MG Raman peak at 1172 cm-1; (d) Normalized SERS intensity of MG at 1172 cm-1 to that of Mpy at 1090 cm-1 versus the MG concentration ranging from 10-10 to 10-6 M. Inset, the magnified plot at low MG concentration ranging from 10-10 to 10-8 M; (e) In-situ detection of MG residue in shrimp tissue. Laser wavelength, 633 nm; Laser power, 2 mW; Integration time, 10 s. The concentration of feeding MG solution was labeled in the figure. The Mpy/DDT-AuNP MLF SERS substrate was further applied for in-situ quantitative detection of MG residue in shrimp. As shown in the inset image in Figure 4e, highly flexible and transparent Mpy/DDT-AuNP MLF/PMMA SERS substrate was conformally attached onto the surface of the shrimp tissue. The Mpy/DDT-AuNP MLF was sandwiched between the PMMA substrate and the shrimp tissue, and the Raman spectrum was measured in back incident mode. The calibration curve between the SERS signal intensity at 1172 cm−1 and the 14


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concentrations of MG were obtained using standard solutions ranging from 10-10 M to 10-7 M. To measure the concentration of the MG residue in a shrimp, a highly flexible and transparent Mpy/DDT-AuNP MLF/PMMA SERS substrate was conformally attached onto the surface of the shrimp tissue. In situ SERS measurement was carried out 1 hour later. The SERS signal was then compared to the calibration curve to determine the MG concentration. As shown in Figure 4e, for the shrimps fed with MG solution with concentration of 10-9, 2 × 10-8, and 10-7 M for 1 hour, the concentration of the MG residue in the shrimp tissue can be calculated from the calibration curve, which were 4 × 10-10, 4 × 10-9, and 1.5 × 10-8 M, respectively. The difference of the measured and the fed concentrations may result from the difference in uptake and in vivo distribution of MG in the shrimp tissue. It is noted that the concentrations of the feeding solutions here are much lower than that for effective insecticidal feeding (100 μg/L, 2.7 × 10-7 mol/L).48 Therefore, our Mpy/DDT-AuNP MLF SERS substrate presents a promising candidate for the direct, noninvasive, and ultrasensitive Raman measurements in food safety and environmental monitoring areas.

4.

CONCLUSION

In summary, we have designed and fabricated a binary thiol-capped AuNP MLF as quantitative SERS substrate. The MLF substrate simultaneously fulfills three key requirements for reliable quantitative SERS analysis: uniform and high-density hotspots, homogenously distributed IS, and evenly adsorbed target molecules. As a result, the MLF substrate has been demonstrated for sensitive and reliable detection of MG over a large concentration range. The excellent SERS performance, low cost, together with high 15



flexibility and transparency, make our SERS substrate a promising candidate for real-time detection of toxic residue in solution or food.

ASSOCIATED CONTENT

Supporting Information

The Supporting Information is available free of charge on the ACS Publications website at DOI: *** Additional preparation information of AuNP colloids; characterization of AuNPs and their MLFs; determination of the absorption saturation time; SERS detection with AuNP MLF substrates (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; [email protected]

Notes

The authors declare no competing financial interest.

ACKNOWLEGEMENT

Y. Fang acknowledge the funding from NSFC (Nos. 21790393 and 21673057).

REFERENCES [1] Cialla, D.; März, A.; Böhme, R.; Theil, F.; Weber, K.; Schmitt, M.; Popp, J. 16


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Binary Thiol-Capped Gold Nanoparticle Monolayer ... - ACS Publications

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