TEMPO-Oxidized Cellulose Nanofiber Based Nanocomposite Papers

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TEMPO-Oxidized Cellulose Nanofiber Based Nanocomposite Papers for Facile in Situ SERS Detection Dabum Kim, Youngsang Ko, Goomin Kwon, Ung-Jin Kim, Jong Hun Lee, and Jungmok You ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.9b03680 • Publication Date (Web): 14 Aug 2019 Downloaded from pubs.acs.org on August 20, 2019

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ACS Sustainable Chemistry & Engineering

TEMPO-oxidized Cellulose Nanofiber Based Nanocomposite Papers for Facile in Situ SERS Detection Dabum Kim†, Youngsang Ko†, Goomin Kwon†, Ung-Jin Kim†, Jong Hun Lee‡*, and Jungmok You†*

†

Department of Plant & Environmental New Resources, Kyung Hee University, 1732

Deogyeong-daero, Giheung-gu, Yongin-si, Gyeonggi-do, 17104, South Korea

‡

Department of Food Science and Biotechnology, College of Life Science, CHA

University, CHA Bio Complex, 335 Pangyo-ro, Bundang-gu, Seongnam-si, Gyeonggido, 13488, South Korea

* Corresponding

authors: Jong Hun Lee and Jungmok You

ACS Paragon Plus Environment

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Keywords: Surface-enhanced Raman scattering (SERS), TEMPO-oxidized cellulose nanofiber, transparent cellulose nanopaper, plasmonic nanocomposite, in-situ detection, pesticide residue.

ABSTRACT

In this study, we report a flexible transparent free-standing surface enhanced Raman scattering (SERS) platform composed of 2,2,6,6-tetramethylpiperidine-1-oxy-oxidized cellulose nanofibers (TEMPO-CNF) and gold nanoparticles (AuNP) such as nanospheres (AuNS) and nanorods (AuNR) for in situ chemical sensing of a real-world surface. The TEMPO-CNF/AuNP-based nanocomposites were fabricated using two-step filtration with pure TEMPO-CNF solution and TEMPO-CNF/AuNP mixture solution. We demonstrate that the TEMPO-CNF/AuNR nanocomposite reveals better SERS activity than the TEMPO-CNF/AuNS nanocomposite. The TEMPO-CNF/AuNR nanocomposite detected rhodamine 6G down to 10 nM with a high enhancement factor of 2.1 × 107 and exhibited


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good SERS measurement reproducibility in the flexible/bent state. No significant change in SERS intensity was observed even after 1000 cycles of bending to nearly 90 degrees. Importantly, the flexible transparent TEMPO-CNF matrix allows the TEMPO-CNF/AuNR nanocomposite to be tightly wrapped onto the surface of an agricultural product for in situ detection as well as to directly detect pesticide residues down to 60 ng/cm2, which is much lower than the maximum residue level for food safety. This high-performance SERS substrate based on flexible transparent nanopaper for rapid in situ detection has great potential in various practical applications such as food safety and environmental monitoring.

INTRODUCTION


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Surface-enhanced Raman scattering (SERS), which is primarily a result of strong electromagnetic field enhancement near metallic nanoparticles, is becoming one of the most widely used spectroscopic tools for the analysis of target analytes because of its high sensitivity and selectivity originating from the unique molecular fingerprints of Raman spectra1-4. In comparison to standard analytical techniques (HPLC, GC-MS, etc.), the most obvious advantage of the SERS technique is ultrasensitive detection without timeconsuming sample pretreatment, which makes it suitable for rapid on-site inspection for trace hazardous molecules. While the rapid advances in both Raman spectrometry and nanofabrication for SERS substrates introduce the SERS technique to real-life applications, there is still a significant challenge to develop high-performance SERSactive substrates with high usability5-11.

Metallic nanoparticle colloids, which are most commonly used for SERS measurement1214,

produce a weakly reproducible signal over large areas owing to their poor stability

originating from aggregation and coagulation. As a result, great efforts have been dedicated to develop two-dimensional SERS substrates based on rigid structures (such


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as silicon and quartz) with high uniformity and good homogeneity for reproducible signals15,16. However, rigid SERS substrates have several intrinsic shortcomings that limit their use in various practical applications. These SERS substrates cannot be directly used for target molecules on irregular and nonplanar surfaces and require extra timeconsuming extraction processes. To overcome these limitations, a number of research groups, including our own, has been pursuing the development of flexible SERS-active substrates, such as cellulose paper, carbon-fiber, and polymers, that can be easily swabbed on curved surfaces, directly collect samples, and thus endow a noninvasive and convenient method of SERS analysis17-21. We recently reported the development of a high-performance SERS substrate based on cellulose nanofibers (CNFs) and gold nanoparticles (AuNPs)22,23. The unique architecture and excellent flexibility of CNF-AuNP nanocomposites have demonstrated great potential for high Raman signal enhancement in SERS swabs for rapid trace detection. Despite their excellent potential for flexible SERS substrates, the optical opacity of SERS substrates including CNF-based nanopapers blocks incident light from direct excitation of SERS on the opposite side, which makes it difficult for use in in situ screening of residues on sample surfaces24-26.


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Thus, flexible transparent free standing SERS substrates recently have drawn considerable attention, since they could enable new functionalities such as in-situ quantification of various samples (liquid, gases, and solids) dispersed on an underlying nonplanar solid surface without any sample pretreatment 5,25.

In this study, we present flexible, transparent, free-standing SERS active substrates with 2,2,6,6-tetramethylpiperidine-1-oxy(TEMPO)-oxidized CNFs and AuNPs of either gold nanospheres (AuNSs) or gold nanorods (AuNRs). Optically transparent TEMPO-oxidized CNF nanopapers are emerging as potential alternatives to conventional substrates (glass, plastic, and general paper) for various applications, such as smart sensors, flexible energy storage, wearable electronics and optoelectronics, as they function as low-cost, bendable, lightweight, and transparent substrates 27-31. We employed both TEMPO-CNFs and AuNPs to fabricate a high performance flexible transparent SERS active substrate. First, we compared the SERS performance of two different types of nanoparticles (AuNPs and AuNRs) embedded on TEMPO-CNF matrix. Then, we investigated whether light can pass easily through the TEMPO-CNF matrix and reach plasmonic nanoparticles to


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connect with analytes, allowing excitation and collection of signals from the backside of the substrate. As an example, we verified that TEMPO-CNF/AuNR nanocomposites directly detected small molecules on real-world surfaces. To the best of our knowledge, this report contains the first example of a transparent nanopaper-based SERS active substrate with high SERS enhancement and excellent bendability. This SERS substrates could serve as a platform of all task substrates in terms of practical applications.

EXPERIMENTAL SECTION

Materials and characterization. Gold(III) chloride trihydrate (HAuCl4), trisodium citrate dehydrate (C6H5Na3O7), cellulose powder (cotton linters), 2,2,6,6-tetramethylpiperidine1-oxy(TEMPO), silver nitrate (AgNO3), sodium borohydride (NaBH4), rhodamine 6G (R6G, fluorescent dye), and thiram (C6H12N2S4, fungicide) were purchased from SigmaAldrich. Cetyltrimethylammonium bromide (CTAB) was purchased from Daejung Chem (Korea). L-ascorbic acid was purchased from Duksan Pure Chemicals Company Co., LTD (Korea). A mixed cellulose ester membrane filter was purchased from Advantec Co., Ltd.


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(Japan). The diameter of AuNSs and the aspect ratio of AuNRs were analyzed by field emission transmission electron microscopy (FE-TEM, JEM-2100 F, JEOL Ltd.). The morphology of the TEMPO-CNF/AuNP nanocomposite was analyzed by a field emission scanning electron microscope (FE-SEM, model SIGMA, Carl Zeiss). The Absorption (transmittance) spectroscopic measurements of AuNSs dispersions, TEMPO-CNF nanopaper, and SERS substrate were performed with a UV-vis spectrophotometer (NeoS2117, Neogen, Korea). The concentration of AuNPs was measured by TGA data from the loading of the AuNPs on nanopaper (data not shown).

Synthesis of gold nanospheres (AuNSs). For synthesis, aqueous solution of HAuCl4 (50 mL, 0.01 wt%) was poured in round-bottom flask with a reflux column. The HAuCl4 solution was heated to boiling while stirring at 700 rpm, followed by the addition of trisodium citrate solution (1mL, 0.5 wt%). The color of total solution turned from faint blue to brilliant red, indicating that the fabrication of gold nanosphere. To complete the synthesis, the solution was boiled for a further 15 min and cooled to room temperature. The concentration of AuNS dispersion was calculated to be 0.004 wt%.


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Synthesis of gold nanorods (AuNRs). HAuCl4 (0.01 M), CTAB (0.1 M), NaBH4 (0.01 M), AgNO3 (0.01 M) and ascorbic acid aqueous solution (0.1 M) were prepared for synthesis of AuNRs, respectively. For seed solution, HAuCl4 (0.25 mL) and CTAB solution (7.5 mL) were mixed by stirring at 200 rpm. After the ice-cold NaBH4 solution (0.6 mL) was added in mixed solution, the total solution was stirred vigorously for 2 min and then aged at room temperature without stirring for 120 min. The color of resulting solution was changed from yellow to pale brown. Growth solution was prepared by sequentially mixing the AgNO3 solution (1.92 mL), CTAB solution (57 mL), HAuCl4 solution (3 mL), and ascorbic acid solution (0.33 mL), followed by the addition of prepared seed solution (72 μL) with stirring for 10 s. The total solution was kept at room temperature for 3 h without stirring. Finally, the excess CTAB in prepared AuNR dispersion was removed by centrifuging (10,000 rpm, 25 min) and washing with distilled water. The concentration of AuNR dispersion was determined to be 0.003 wt%.

Fabrication of the TEMPO-oxidized cellulose nanofibers (TEMPO-CNFs). To prepare the TEMPO-CNFs dispersions, the cellulose powder (1 g) was dispersed in 100 mL water


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containing TEMPO (0.016 g) and NaBr (0.1 g). The pH value of 12% NaClO solution was regulated to pH 10 using 0.1 M hydrochloric acid. The TEMPO-mediated oxidation was conducted by adding the prepared NaClO solution (5 mmol) to cellulose fiber dispersion while stirring. The 0.5 M NaOH was continuously dropped into a mixture solution to maintain the pH 10 until the pH change in the solution was not observed. After oxidization, TEMPO-CNFs dispersion was centrifuged at 4,000 rpm for 20 min and dispersed again in distilled water. Finally, the TEMPO-CNF dispersion was nano-fibrillated by a highpressure homogenizer (Nano Disperser-NLM 100, Ilshin Autoclave Co. Ltd., 10 passes at 1000 bar). The concentration of TEMPO-CNF dispersion was 0.06 wt%.

Fabrication of TEMPO-CNF/AuNR nanocomposites. To fabricate the TEMPO-CNF nanopaper matrix, the vacuum filtration was performed by pouring the TEMPO-CNF dispersion (60 mL) into a cellulose ester membrane (0.2 μm pore size, 47mm diameter). To construct the TEMPO-CNF/AuNP nanocomposite for SERS analysis, the various ratio of mixture (TEMPO-CNF and AuNSs) was filtered into the non-dried TEMPO-CNF matrix.


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After completely dried at 70 °C for 3 h, the resultant TEMPO-CNF/AuNP-based SERS active substrate was peeled off the filter membrane.

SERS measurements. A spectrometer (QEPRO, Ocean Optics Inc.) with 785 nm laser was used for SERS measurement. To measure the performance of substrate for SERS, 5 μL of analyte solution (R6G, thiram) of varying concentrations were dropped on the SERS substrates. SERS signals were directly obtained from both front and back side illumination of TEMPO-CNF/AuNS nanocomposites. The SERS enhancement factor was defined in the same way described in previous literature22.

Bending and stability test of TEMPO-CNF/AuNR nanocomposites. In the bending test, the SERS signal of R6G (10 μM) was measured with 10, 30, 50, 70, 90% bent TEMPOCNF/AuNR nanocomposites (1.5 cm × 1.5 cm). In the bending cycle test, the TEMPOCNF/AuNR nanocomposites was repeatedly bent and unbent with a strain of 90% for 1,000 cycles. In the stability test, TEMPO-CNF/AuNR nanocomposites dropped with R6G (10 μM) were stored in the cabinet of the laboratory. The measurement of SERS signals


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of R6G (10 μM) was conducted every day and then compared with SERS signals obtained from first day.

Direct SERS measurement on real-world surfaces (apple peel). For direct thiram detection, thiram solutions (EtOH) with different concentrations ranging from 60 to 1200 ng/cm2 were dropped onto apple samples and then completely dried at room temperature. For in-situ SERS measurement, 5 μL of pure ethanol solution was dropped on the thiramdried area and then gently covered with an upside-down SERS substrate for backside SERS analysis. After drying, a SERS laser was directly exposed to the apple peel covered with the SERS substrate.

RESULTS AND DISCUSSION

Fabrication and characterization of TEMPO-CNF/AuNP nanocomposites. Fabrication of TEMPO-CNF/AuNP nanocomposites via two-step vacuum filtration is schematically illustrated in Figure 1A32. Briefly, TEMPO-CNF solution was filtered on a filter membrane to prepare the flexible transparent matrix, and then the mixture solution of TEMPO-CNF and AuNP (AuNS or AuNR) was filtered through a TEMPO-CNF matrix. Importantly, in the second filtration, TEMPO-


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CNFs were used to trap AuNPs and increase the interfacial adhesion between TEMPO-CNF matrix and plasmonic layer (discussed later in Figure 2). After complete drying, the resultant TEMPOCNF/AuNP nanocomposites were peeled off the filter membrane to produce flexible, transparent SERS active substrates. It is worth noting that TEMPO-CNF/AuNP nanocomposites can be easily fabricated via simple and low-cost vacuum-assisted filtration without any expensive instrument. In our previous study, the carboxylate contents on the TEMPO-CNF, which were formed by selective oxidation of C6 primary hydroxyl groups, was obtained to be 0.3 mmol/g by conductometric titration, and the zeta-potential value at the TEMPO-CNF surface was found to be -33 mV27. The presence of electrostatic repulsion is responsible for the formation of individual TEMPO-CNF, resulting in a more stable dispersion (Figure 1A) as well as highly optical transparent nanopaper with a T550 nm around 86% (Figure 1E). The reason for high transparency of TEMPO-CNF matrix is that the smaller TEMPO-CNFs with a diameter of 20-30 nm form a tighter structure that reduces light scattering significantly compared to opaque cellulose paper and cellulose nanopaper (39 % transmittance at 550 nm) made of only mechanically treated CNFs 27,33. Figures 1B and C exhibit FE-TEM images of the AuNSs with a diameter of 41 nm and AuNRs with an aspect ratio of 4.0 (28.4±2.5 × 7.1±0.5 nm), which were used separately as plasmonic nanoparticles for SERS active substrates.


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Figure 1. (A) Fabrication of TEMPO-oxidized cellulose nanofiber (TEMPO-CNF)/AuNP (AuNS or AuNR)-based SERS active substrates using two-step vacuum-assisted filtration: (i) 1st filtration of the TEMPO-CNF solution on the membrane filter to produce the transparent TEMPO-CNF matrix; (ii) 2nd filtration of the mixture solution of TEMPOCNF and AuNP on TEMPO-CNF matrix to produce flexible, transparent nanopaper-based SERS substrates. (B and C) HR-TEM images of AuNSs and AuNRs, scale bar = 50 nm. (D) UV-vis spectra of AuNS dispersion, AuNR suspension, TEMPO-CNF/AuNS nanocomposite, and TEMPO-CNF/AuNR nanocomposite. (E) Optical transmittance of the TEMPO-CNF matrix. Inset: Photographic image of the transparent TEMPO-CNF matrix (3cm × 3cm) and chemical structure of TEMPO-CNF. (F) Photographic and (G) top-view/cross-sectional FE-SEM images of TEMPO-CNF/AuNR nanocomposite substrate (3cm × 3cm).

As shown in the UV-vis spectra (Figure 1D), one strong absorption peak of the AuNS solution is observed at 532 nm, and two absorption peaks of the AuNR solution appear at 525 nm and 748 nm, corresponding to the transverse and longitudinal bands of AuNRs,


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respectively34. After filtration of the AuNP solution through TEMPO-CNF matrix, the UVvis spectra of TEMPO-CNF/AuNP nanocomposites are slightly red-shifted and broader than those of the AuNP solution. These results are mainly ascribed to plasmonic coupling between adjacent AuNPs such as AuNPs and AuNRs35. As shown in the photographic and top-view/cross-sectional FE-SEM images (Figures 1F and G), AuNPs were homogeneously distributed and embedded in the TEMPO-CNF matrix, resulting in a uniform plasmonic layer.

The stability of the TEMPO-CNF/AuNP nanocomposite is of great significance in practical application as a SERS substrate. We examined the adhesive strength of plasmonic layer on TEMPO-CNF matrix. Unlike our previous methods for fabricating SERS substrates22,23, the plasmonic layer was easily detached from the TEMPO-CNF matrix by rubbing with a cotton swab when only AuNPs were loaded on the TEMPO-CNF matrix via vacuum filtration (Figure 2A). Detachment of AuNPs resulted in loss of the SERS signals of R6G (Figure 2A). These observations may be explained by poor interfacial adhesion between the TEMPO-CNF matrix and AuNP layer.


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Figure 2. Comparison of the adhesive stability of the TEMPO-CNF/AuNP nanocomposites fabricated by two different approaches in the second filtration step: (A) pure AuNP solution was filtered through the TEMPO-CNF matrix, and (B) the mixture of TEMPO-CNF and AuNP solution was filtered through the TEMPO-CNF matrix.

In contrast, Figure 2B clearly shows that the plasmonic layer was stable after rubbing when the mixture of TEMPO-CNFs and AuNPs solution was loaded on the TEMPO-CNF matrix via vacuum filtration (Figure 1A), indicating strong adhesion to the TEMPO-CNF matrix. The SERS signals of the R6G nanocomposite was stable after rubbing (Figure


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2B). It is important to note that TEMPO-CNFs serve as a stable frame to fix AuNPs inside plasmonic layer. If only AuNPs without TMEPO-CNFs were loaded on top of the TEMPOCNF matrix, the AuNPs were unable to form a stable plasmonic layer. The SERS signals of rhodamine 6G (R6G, 10 μM), a reference molecule to determine SERS activity, were compared to optimize the mixture ratio of TEMPO-CNF and AuNPs (Figure S1). The highest SERS intensity of R6G at 613 cm-1 (aromatic C-C-C bending) and 1359 cm-1 (aromatic C-C stretching) was observed for a 0.2:1 mass ratio of TEMPO-CNF (0.3 mL, 0.06 wt%) and AuNS (18 mL, 0.0045 wt%) in mixture solution.

SERS performance of TEMPO-CNF/AuNP or TEMPO-AuNR nanocomposites. The SERS activity of plasmonic NPs is highly dependent on their shape and spatial distribution, which are strongly associated with plasmon resonance properties. Thus, we first optimized the SERS activity of two different SERS substrates (TEMPO-CNF/AuNS and TEMPO/AuNR) by changing the volume of feed solution at filtration and then compared the SERS activity of these substrates. In our previous studies, we confirmed


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that vacuum filtration enables high loading of metal nanostructures on nanopaper without penetration because the pore size in the nanopaper is much smaller than the size of the metal nanostructures.

Figure 3. (A) SERS intensity of R6G molecules (10 μM) on the TEMPO-CNF/AuNS nanocomposite as a function of volume of AuNS feed solution (0.0045 wt%). (B) SERS spectra of R6G molecules (100 μM – 1 μM) on TEMPO-CNF /AuNS substrate. (C) SERS spectra obtained from both sides (front or back excitation) of the TEMPO-CNF/AuNS substrate.

As shown in Figure 3A and Figure 4A, the SERS intensity of R6G gradually increased as the volume of AuNS (0.0045 wt%) and AuNR (0.0027 wt%) solution increased to 18 mL and 14 mL, respectively, while the SERS intensity decreased at higher volumes of AuNS (22 mL) and AuNR (14 mL) solutions. The lower SERS activity might be due to reduction of plasmonic hot spots originating from high loading of AuNPs. Therefore, 18 mL AuNS


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solution and 14 mL AuNR solution were chosen for further experiments related to the detection sensitivity of TEMPO-CNF/AuNS and TEMPO-CNF/AuNR nanocomposites. Figure 3B and Figure 4B show the SERS spectra of R6G with various concentrations on TEMPO-CNF/AuNS and TEMPO-CNF/AuNR, respectively. The representative R6G peaks at 613 cm-1 and 1359 cm-1 were detectable on the TEMPO-CNF/AuNS substrate at a concentration as low as 1 μm, while representative R6G peaks at 613 cm-1 and 1508 cm-1 were clearly visible on the TEMPO-CNF/AuNR substrate even at a concentration of 10 nM. There results demonstrate that the TEMPO-CNF/AuNR substrate is approximately 100 times more sensitive than the TEMPO-CNF/AuNS substrate. In addition, the Raman enhancement factors (EFs) of TEMPO-CNF/AuNS and TEMPO-CNF/AuNR substrates were 8.6 × 105 and 2.1 × 107, respectively, indicating that AuNRs greatly enhance the Raman signals on TEMPO-CNF matrix compared to AuNSs. The SERS EF value of TEMPO-CNF/AuNR nanocomposite is comparable to SERS sensitivity (EF: 1 × 104 ~ 5 ×

106)

previously

reported

for

transparent

SERS

substrates

with

high

performance19,21,25,26,36-41. These results might be due to precise control of AuNR


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distribution as well as stronger field enhancement of AuNRs related to higher SERS efficiency42-44.

Figure 4. (A) SERS intensity of R6G molecules (10 μM) on TEMPO-CNF/AuNR nanocomposite as a function of volume of AuNR feed solution (0.0027 wt%). (B) SERS spectra of R6G molecules (100 μM – 10 nM) on TEMPO-CNF/AuNR substrates. (C) SERS spectra from both sides (front or back excitation) of the TEMPO-CNF/AuNR substrates.

Importantly, SERS signals were obtained from both sides (front excitation and back excitation) of the TEMPO-CNF-based SERS substrates as the TEMPO-CNF matrix possesses excellent optical transparency. As shown in Figure 3C and Figure 4C, the SERS intensities collected from the two sides of TEMPO-CNF/AuNS and TEMPOCNF/AuNR were nearly identical at the lowest detectable concentration of R6G. To further


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investigate the signal reproducibility of TEMPO-CNF/AuNR, we analyzed the SERS signals collected from 40 randomly selected positions. As shown in Figure S2, the relative standard deviation (RSD) of the signal intensities at 1508 cm-1 was 8.26% with front excitation and 9.60% with back excitation.

To evaluate the flexibility and bendability of the resultant TEMPO-CNF/AuNR nanocomposite, we analyzed the Raman intensity of R6G at 1508 cm-1 under different bending strain (10 – 90%) as well as during more than 1,000 bending cycles at a radius of 1 mm. As shown in Figure 5A and B, there were no obvious changes in Raman intensities even though TEMPO-CNF/AuNR not only underwent a bending strain of 90%, but also bent 1,000 times to nearly 180 degrees. These results strongly suggest that physical forces, such as bending, have little impact on the SERS performance of TEMPOCNF/AuNR nanocomposite. Compared to other flexible transparent SERS substrates based

on

polydimethylsiloxane

(PDMS)

elastomer19,36,37,38,45

and

polyethylene

terephthalate film (PET) matrix21,41, TEMPO-CNF/AuNR nanocomposites shows excellent mechanical stability because cellulose paper with hydrogen-bonding network


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structure is intrinsically flexible and bendable. To further investigate the stability of TEMPO-CNF/AuNR composite in the aspect of SERS performance, we analyzed the Raman intensity at R6G at 1508 cm-1 as a function of storage time in air. Figure 5C clearly shows that SERS intensity remained unchanged over 5 days, indicating that TEMPOCNF/AuNR has a good stability for the performance in air condition.

Figure 5. Changes in SERS intensity (at 1508 cm-1, R6G 10 μM) of the TEMPO-CNF/AuNR nanocomposite dependent on (A) bending strain and (B) bending cycles. The SERS substrate underwent bending around a radius of 1 – 7 mm (around strain 10 – 90%). (C) Changes in SERS intensity (at 1508 cm-1, R6G 10 μM) of the TEMPO-CNF/AuNR nanocomposite as a function of storage time in air.

Application of TEMPO-CNF/AuNR nanocomposite for direct detection of pesticide residue from a real-world surface. We demonstrate the practical application of TEMPOCNF-based SERS substrates for detection of residual pesticide, which is a major public


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health concern46. Thiram, a dithiocarbamate compound, is one of the most commonly used fungicides for protection of fruits and vegetables and so was analyzed here47. Figure 6 exhibits the SERS spectra of thiram at various concentrations on both TEMPOCNF/AuNS and TEMPO-CNF/AuNR nanocomposites. The prominent Raman peaks for thiram clearly appeared at 432 cm-1 (CH3NC deformation and C=S stretching), 568 cm-1 (S-S stretching), and 1386 cm-1 (CN stretching and CH3 deformation). The lowest concentration visually detectable for thiram on TEMPO-CNF/AuNP and TEMPOCNF/AuNR was 1 μM and 100 nm, respectively. Similar to detection of R6G, TEMPOCNF/AuNR has greater sensitivity for thiram than TEMPO-CNF/AuNS.


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Figure 6. SERS spectra of thiram at different concentration on (A and B) TEMPO-CNF/AuNS and (C, D) TEMPOCNF/AuNR nanocomposites.

Importantly, SERS signals collected from the back sides of TEMPO-CNF/AuNS and TEMPO-CNF/AuNR were clearly observed at the lowest detectable concentrations of thiram (Figures 6B and D).


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Additionally,

we

demonstrated

the

practical

feasibility

of

TEMPO-CNF/AuNR

nanocomposite for in situ detection of thiram on real-world objects. The advantages of paper-based SERS substrates is the ability to perform a SERS swab for direct and rapid detection. Thus, we investigated the SERS spectra of thiram residue on an apple surface obtained by swabbing slightly wetted TEMPO-CNF/AuNR across the apple surface with different residue levels of thiram (Figure S3). The strongest Raman peaks at 432 cm-1 and 1386 cm-1 were detectable down to 60 ng/cm2 on the surface.

Figure 7. (A) In situ detection of thiram on the apple surface through back excitation. (B) SERS spectra of thiram residues with different concentrations (60 – 1200 ng/cm2) on the TEMPO-CNF/AuNR nanocomposite.


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More importantly, we confirmed collection of SERS signals from TEMPO-CNF/AuNR nanocomposite directly applied onto apple surfaces via back-excitation since TEMPOCNF matrix is mechanically flexible and optically transparent. As shown in Figure 7A, the TEMPO-CNF/AuNR nanocomposite was cut into small pieces, slightly wetted with 5 μL of an ethanol solution, and then directly applied onto the apple surfaces contaminated with thiram. As a result, thiram residues were successfully detected on the TEMPO-CNFAuNR nanocomposite (Figure 7B). The strongest peaks at 432 cm-1 and 1386 cm-1 were distinguished even with a thiram concentration as low as 60 ng/cm2. This was because the TEMPO-CNF/AuNR nanocomposite closely contacted thiram residues on apple surfaces and significantly amplified the Raman signals of thiram, even with backexcitation. The lowest detection concentrations of thiram on TEMPO-CNF/AuNR via a SERS swab or direct in-situ detection (back-excitation) were significantly lower than the maximum residue level (MRL) of 2000 ng/cm2

48,49.

Considering the low-cost, easy

fabrication, direct signal collection, and high sensitivity of the TEMPO-CNF/AuNR


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nanocomposite, this transparent nanopaper-based SERS substrate can serve as a promising SERS platform for various practical applications.

CONCLUSIONS

In summary, we firstly developed a flexible transparent nanopaper-based SERS substrate composed of TEMPO-CNFs and AuNPs. The flexible transparent TEMPO-CNF matrix has great potential and advantages in serving as a SERS substrate, enabling direct insitu and nondestructive detection of pesticide residues on the surface. The TEMPOCNF/AuNR nanocomposite exhibited good adhesion stability, excellent mechanical bendability, and high sensitivity of SERS performance. Importantly, we demonstrated that the TEMPO-CNF/AuNR nanocomposite directly applied to an apple surface could detect pesticide residues as low as 60 ng/cm2. We anticipate that our transparent paper-based SERS substrates will bring SERS analysis closer to practical application and be valuable in development of flexible and transparent sensors.


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ASSOCIATED CONTENT

Supporting Information

Optimizing mass ratio of TEMPO-CNF and AuNS in mixture solution, reproducibility of SERS signals, swab detection of SERS substrates.

AUTHOR INFORMATION

Corresponding Author *Phone: +82-31-201-2626 *E-mail: [email protected] and [email protected]

ORCID Jong Hun Lee: 0000-0002-3083-7595 Jungmok You: 0000-0002-9583-2242

Notes


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The authors declare no competing financial interest.

ACKNOWLEDGMENT This study was supported by a Cooperative Research Program for Agriculture Science and Technology Development (No. PJ0127972019) Rural Development Administration, a National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIP) (No. 2018R1D1A1B07047874), and a Research/Publication Support Program funded by Ottogi Ham Taiho Foundation (No. 2019).

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For Table of Contents Use Only


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TOC. Flexible transparent TEMPO-CNF/AuNR nanocomposite paper for Facile in Situ SERS Detection.


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TEMPO-Oxidized Cellulose Nanofiber Based Nanocomposite Papers

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