Polydopamine-Assisted In Situ Growth of AgNPs on Face Masks for the Detection of Pesticide Based on Surface-Enhanced Raman Scattering Spectroscopy

Thiram is used as a fungicide and insecticide in agriculture to prevent white rot and anthracnose, but it also produces certain pesticide residues that endanger human health. Polydopamine (PDA) is an adhesive polymer that functionalizes almost all chemical materials, having been inspired by the adhesive properties of catechol and amines in mussel adhesive protein. In this work, we have fabricated a highly sensitive face mask (FM)–based surface-enhanced Raman scattering (SERS) substrate by depositing silver nanoparticles (AgNPs), using a PDA layer as an interface to promote adhesion. Face masks are currently ubiquitous, and are simple and easy to obtain, and this approach can be regarded as waste utilization. FM/PDA/AgNPs can detect Nile blue A (NBA) probe molecules at concentrations as low as 1.0 × 10−9 M, and permit measurements with sensitivity and uniformity. The Raman substrate can be used to swab the surface of fruit to detect thiram at concentrations as low as 1.0 × 10−7 M, and the reproducibility has been assessed. The simple utilization of this flexible SERS substrate holds promise for its practical application in the field of pesticide control.


Introduction
Research has proved that the molecular structures of pesticides determine their stability and persistence, and most of them are difficult to excrete from the human body through the digestive system. Pesticide residues in fruits and vegetables cause chronic poisoning and can induce many chronic diseases. Thiram is a representative dithiocarbamate (DTC) fungicide with a strong protective effect, which can control white rot and anthracnose in fruit. Thiram should be used rationally; if it is widely used with other DTC pesticides, it can elicit nervous system, endocrine-disrupting, and carcinogenic effects, raising concerns for human health [1].
Pesticide residues are very harmful, and even at trace levels can increase physical burden and even severely disrupt ecosystems [2,3]. The development of strategies to monitor the application of pesticides is of primary importance.
Face masks (FM) have become more and more important part of daily life. Here, we have successfully prepared a polydopamine (PDA)/silver nanoparticle (AgNPs) substrate on FM. Dopamine self-polymerizes in an alkaline environment to form a PDA layer bearing a large number of phenolic hydroxyl and amino functional groups, which can bind silver ions. Furthermore, PDA has some reducing ability, and can reduce silver ions to silver nanoparticles. The FM/PDA/ AgNP substrate displays high uniformity and sensitivity for the detection of Nile blue A (NBA) molecules. In addition, in order to verify its practical application and reproducibility on various curved surfaces, the FM/PDA/AgNP substrate has been used to detect thiram residues on fruits. The results prove that the substrate can realize waste utilization, is characterized by a simple preparation method, low production cost, high sensitivity, high uniformity, and reproducibility, and provides a potential platform for efficient and flexible SERS sensors for use in biochemical identification.

Materials
Dopamine hydrochloride, silver nitrate (AgNO 3 ), Tris-base, hydrochloric acid, and NBA were purchased from Energy Chemical Co., Ltd. (Anhui, China). Thiram was purchased from Aladdin Industrial Co. (Shanghai, China). Ethanol and acetone were purchased from Tianjin Yongda Chemical . Other chemicals were of analytical grade or high reagent grade. The FM (size 9.5 × 17 cm ± 10%; manufacturer Henan Yadu Industrial Co., Ltd.; composition non-woven fabric and polypropylene melted cloth) and fruit were purchased from a local supermarket (Shenyang, China). There were three layers after the FM had been cut, and the middle layer of polypropylene melted cloth was used.

Preparation of PDA/AgNPs on FM
A schematic diagram of the preparation of PDA/AgNPs on a FM is shown in Fig. 1. The FM was first ultrasonically cleaned with a sequence of ethanol, acetone, and deionized (DI) water in order to remove other substances that may have been present on its surface and to ensure that subsequent experimental operations were in a dust-free environment. It was then dried in an oven at 50 ℃. Subsequently, the clean FM was dipped in Tris-HCl buffer solution containing dopamine (2 g/L) and placed in the dark for 24 h. Due to spontaneous oxidative polymerization, a PDA-modified FM was obtained. Finally, the FM was thoroughly washed with DI water and dried in a nitrogen atmosphere. The obtained composite material is denoted as FM/PDA. After gentle washing with ethanol, acetone, and DI water, the FM/PDA was dried in an oven at 50 ℃. In order to achieve uniform growth of AgNPs on its surface, the obtained FM/PDA substrate was placed in a fresh [Ag(NH 3 ) 2 ] + solution and stirred for 12 h at room temperature. Because of the adsorption and reducing ability of PDA molecules, especially towards silver ions, AgNPs were preferentially deposited on the FM/PDA surface. After several hours of reaction, uniform substrates were produced. The as-prepared substrates obtained after washing with DI water are denoted as FM/PDA/AgNPs.

Characterization
The morphological characteristics of FM/PDA/AgNPs were observed by means of a scanning electron microscope (SEM, JEOL, JSM-7400 V, Japan) operated at an accelerating voltage of 5.0 kV. X-ray photoelectron spectra (XPS) were recorded on a Thermo Scientific ESCALab XI + spectrometer (Thermo Fisher, USA). Raman spectra were collected at room temperature (20 ℃) with a Renishaw 2000 Raman spectrometer (Renishaw plc, Wotton-under-Edge, UK) using an excitation source with λ = 532 nm. The band of a silicon wafer at 520 cm −1 was used to calibrate the spectrometer. SERS spectra were acquired through a × 50 objective lens, and data were processed using LabSpec software.

SERS Performance Test
To evaluate the sensitivity of the FM/PDA/AgNPs substrate, an ethanolic solution of NBA was diluted from 10 −3 to 10 −9 M. Aliquots (2 μL) of ethanolic NBA solutions of different concentrations were dropped on the substrate and allowed to dry naturally.

Direct Detection of Pesticide Residues
Fruits were thoroughly washed with DI water. Aliquots (10 μL) of ethanolic thiram solutions at concentrations ranging from 10 −3 to 10 −7 M were then sprayed thereon and allowed to dry naturally. The spraying area was restricted to 1 × 1 cm 2 . Then, the FM/PDA/AgNP substrate was used to carefully swab the thiram on the surface of the fruit.

Morphological Characterization
SEM images were acquired to inspect the morphology of the FM at different magnifications, as shown in Fig. 2a, b. When the substrate was modified by dopamine, its original smooth and flat surface became rough and uneven, indicating successful self-polymerization of the dopamine (Fig. 2c,  d). Figure 2e, f shows SEM images of FM/PDA/AgNPs. It can be seen that AgNPs were uniformly distributed on the surface of the PDA-modified FM, resulting in a large number of nanostructures and nano gaps, forming a large number of SERS hot spots.

Elemental and Structural Analysis
XPS was used to analyze the elemental composition and valence state changes according to the positions of the characteristic lines in the energy spectrum. As can be seen in Fig. 3a, the wide-scan XPS pattern of the original FM features two peaks due to C and O, whereas that of the FM/PDA/AgNPs substrate (Fig. 3b) features four peaks due to C, O, N, and Ag, indicating successful assembly of the substrate. In Fig. 3c, the N 1 s signal at 400 eV can be attributed to the NH 2 and NH groups in PDA [44]. The high-resolution XPS pattern of Ag 3d (Fig. 3d) features Ag 3d 5/2 and Ag 3d 3/2 signals at binding energies of 368.2 eV and 374.2 eV, respectively. Additionally, the splitting of the 3d doublet is 6.0 eV, indicating the presence of the Ag 0 state in FM/PDA/AgNPs [45].

SERS Performance
The SERS performance of the FM/PDA/AgNP substrate was evaluated using NBA as a probe molecule. Figure 4a shows the SERS spectra of NBA adsorbed on the substrate at a series of concentrations (10 −3 -10 −9 M). When the concentration was as low as 10 −9 M, the SERS intensity (at 591 cm −1 for NBA) could still be clearly discerned [22]. This high sensitivity can probably be ascribed to effective hot spots on the FM/PDA/ AgNP substrate. Figure 4b shows the relationship between SERS intensity at 591 cm −1 and the logarithm of NBA concentration. The linear correlation coefficient is 0.990, which holds promise for accurate quantification of research targets. To examine the uniformity of the FM/PDA/AgNP substrate, 16 random locations were selected for the recording of SERS signals from NBA, as shown in Fig. 4c. The relative standard deviation (RSD) of the peak intensity at 591 cm −1 within one FM/PDA/AgNP substrate was 14.12% (Fig. 4d), indicating acceptable homogeneity.

Application in On-Site Detection
Swabbing is considered to be the most versatile sampling method, as it can be used to analyze target molecules from surfaces of various shapes [14][15][16][17][18][19][20][21][22]29]. Therefore, the FM/PDA/AgNP substrate was used to detect thiram residues on the surfaces of grapes and pears. Aliquots (10 μL) of ethanolic thiram solutions of different concentrations were sprayed on the surfaces of the fruit and allowed to dry naturally in air. The substrate was then swabbed on the target surface. As the concentration of thiram was increased, its characteristic peaks could be easily observed. As shown in Fig. 5a, b, Raman bands at 568 cm −1 (S-S stretching), 1145 cm −1 (S-C-N stretching, CH 3 rocking), and 1390 cm −1 (C-N stretching, CH 3 rocking) were clearly apparent in the spectra. The 1390 cm −1 peak could be used as an analytical readout to assay the residues on the surfaces of grapes and pears, as has been reported previously [46]. With decreasing thiram concentration, the SERS intensity of the characteristic peak decreased accordingly. Nevertheless, even when the thiram concentration was as low as 10 −7 M, its characteristic peaks could still be observed. Figure 5c, d shows that the SERS peak intensity (measured at 1390 cm −1 ) conformed to a linear dependence on the concentration of thiram, which is the basis for quantitative spectral analysis. The results indicate reliable SERS quantification, with regression coefficients (R 2 ) of 0.988 and 0.910, respectively. By spraying an aliquot (10 μL) of a 1 × 10 −7 M thiram pesticide solution on the surface of a grape, about 0.24 ng of thiram deposition can be estimated for an area of 1 × 1 cm 2 , and the Raman intensities of the analyte at this level were recorded by the direct swabbing method. According to the above quantitative method, thiram residues on the surface of pears were likewise detected, and the detectable amount was as low as 0.48 ng/cm 2 . The results show that the FM/PDA/AgNPs substrate has high SERS sensitivity, and that thiram residues can be detected on various surfaces.
To further explore the reproducibility of the FM/PDA/AgNP substrate for the direct detection of pesticides on fruit surfaces, ten different grape surfaces bearing the same concentration of 2.40 ng/cm 2 were swabbed for residue determination. As shown in Fig. 6a, the characteristic band at 1390 cm −1 was consistently recorded. The Raman intensity fluctuations of thiram from the different grape surfaces were small, with a calculated RSD (n = 10) of 17.66% from the statistical results. This indicated that the FM/PDA/AgNP substrate had high reproducibility in practical analysis. These results make the FM/PDA/AgNP substrate promising for practical applications.

Conclusions
In summary, inspired by an adhesive interaction in mussels, AgNPs have been grown in situ on a PDA-modified FM. In this way, an FM/PDA/AgNP substrate with high sensitivity, uniformity, and reproducibility was obtained. FM are currently ubiquitous, and their further use can be regarded as waste utilization. The obtained substrate can be used to detect NBA probe molecules at concentrations as low as 1.0 × 10 −9 M. The flexible FM/ PDA/AgNP substrate could be used to detect different concentrations of thiram pesticide on the surface of fruits by swabbing. Swabbing of ten different fruits with the substrate demonstrated good reproducibility. The FM/PDA/AgNP substrate is flexible, making it very suitable for sampling and testing real samples. In addition, the simple preparation method can meet the growing demand for actual analysis and endows the substrate with great potential in the fields of environmental and biological sciences.
Author Contribution All authors contributed to the study conception and design. Material preparation, data collection, and analysis were performed by Tongtong Wang, Qijia Zhang, Jia Li, Guangda Xu, Na Guo, Peng Song, and Lixin Xia. The first draft of the manuscript was written by Tongtong Wang and all authors commented on previous versions of the manuscript. All authors read and approved the final manuscript. Availability of Data and Material All data generated or analyzed during this study are included in this published article.

Declarations
Ethics Approval Not applicable.

Consent for Publication Not applicable.
Competing Interests The authors declare competing interests.