Ionic Liquid-Assisted Thermal Evaporation of Bimetallic Ag–Au Nanoparticle Films as a Highly Reproducible SERS Substrate for Sensitive Nanoplastic Detection in Complex Environments

Nanoplastic particles are emerging as an important class of environmental pollutants in the atmosphere that have adverse effects on our ecosystems and human health. While many methods have been developed to quantitatively detect nanoplastics; however, sensitive detection at low concentrations in a complex environment remains elusive. Herein, we demonstrate a greener method to fabricate a surface-enhanced Raman spectroscopy (SERS) substrate consisting of self-assembled plasmonic Ag–Au bimetallic nanoparticle (NP) films for quantitative SERS detection of nanoplastics in complex media. The self-assembly of Ag–Au bimetallic NPs was achieved through thermal evaporation onto a vapor-phase compatible ionic liquid based on deep eutectic solvent over the growth substrate. The finite-difference time-domain simulation revealed that the localized field enhancement is strong in the gaps, which generate uniform SERS “hotspots” in the obtained substrate. Benefiting from highly accessible SERS “hotspots” at the gaps, the SERS substrate exhibits excellent sensitivity for detecting crystal violet with a limit of detection (LOD) as low as 10–14 M and excellent reproducibility (RSD of 5.8%). The SERS substrate is capable of detecting PET nanoplastics with LOD as low as 1 μg/mL and about 100 μg/mL in real samples such as tap water, lake water, diluted milk, and wine. Moreover, we also validated the feasibility of the designed SERS substrate for the practical detection of PET nanoplastics collected from commercial drinking water bottles, and it showed great potential applications for sensitive detection in actual environments.


SERS Enhance Factor (EF) estimation:
The estimation of SERS enhancement factor (EF) was estimated based on following the previous studies. 1The SERS EFs for the CV covered substrates were calculated using the following relation: -

Chemical Synthesis of gold nanoparticles and Au-Ag bimetallic nanoparticles and SERS Analysis:
The spherical Au NPs with particle sizes of approximately 10 nm, and Au-Ag core-shell NPs and nanocubes were synthesized by following earlier report. 2The formation of spherical Au NPs, Au-Ag, and Ag-Au core-shell NPs was confirmed using UV-vis and TEM imaging (Figure S8).Then, 50 µL of cleaned colloidal dispersion was deposited over glass substrate through drop casting, and allowed to dry to obtain SERS substrate based on Au NPs and Au-Ag NCs.After that, 20 µL of CV (10-6 M) was deposited over the substrate and SERS spectra were recorded.

Figure
Figure S2.a, b) Typical SEM images of Ag-Au films obtained without utilizing DES onto growth substrate (glass).Deposition pressure of 2x10 -4 mbar, applied current of 4 amperes.

Figure S5 .
Figure S5.Typical SERS spectra of Rhodamine 6G (R6G) onto Ag-Au SERS substrates.a) SERS spectra of R6G onto Ag-Au on glass substrate obtained at different thermal evaporation pressures b) corresponding SERS peak intensities at 1312, 1365, and 1510 cm - 1 , respectively.c) SERS spectra of R6G onto Ag-Au NPs films obtained at varied Ag/Au ratios, and d) corresponding peak intensities of peaks at 1312, 1365, and 1510 cm -1 , respectively.
----------(1) Where, ISERS and INor are the signal intensities of SERS and normal Raman spectra of CV for the same dispersion band (1620 cm -1 ), and NSERS and NNor represent the corresponding number of molecules in the focused incident laser spot.Assuming a uniform distribution of CV molecules over the substrates, the values of NSERS and NNor in eq.1 can be substituted by the concentration of CV, that is, 1.0 × 10 -6 and 1.0 × 10 -3 M, respectively.

Figure S6 .Figure
Figure S6.Comparison of the SERS enhancement factors values for different substrates.a) Ag-Au NPs film substrates obtained by varying the deposition pressure b) Ag-Au NPs films obtained by varying Ag/Au ratio.

Figure S8 :
Figure S8: a) UV-vis spectra of Au and Au-Ag NPs.b, c, d) STEM images of Au NPs, Au-Ag NPs, and Au-Ag nanocubes, e) comparison of SERS spectra of CV onto the Au NPs, Au-Ag NPs and Au-Ag nanocubes.D) corresponding Raman peak intensity of CV at 1620 cm -1 for different SERS substrates.

Figure S9 .
Figure S9.SERS spectra of CV (1x10 -6 M) on 25 different sports in the same substrate of Ag-Au (1:2) p4 substrates, b) corresponding signal intensities of peak at 1620 cm -1 vs number of spots.

Figure S11 .
Figure S11.Stability of the self-assembled Ag-Au film substrates.a) SERS spectra of CV (10 -6 M) covered on self-assembled Ag-Au SERS substrate collected in each 2-days for 10 days period.b) The variation of SERS peak intensity at 120 cm -1 as a function of storage time (days).

Figure
Figure S13.a) AFM topographical images of PET nanoplastic spheres onto the Ag-Au NPs film substrate, b) AFM height profile analysis of PET particles over the substrate.

Figure S14 :
Figure S14: a, b) AFM images of polystyrene nanospheres (size=100 nm) onto selfassembled Ag-Au NPs film substrate.c) SERS spectra of PS nanospheres with different concentration over Ag-Au NPs film substrate, d) corresponding linear calibration plot of Raman intensity at 1001 cm -1 vs. PS nanosphere concentration.e) Raman images of PS nanospheres containing real-samples, f) SERS spectra of PS nanospheres (10 mg/mL) containing real-samples.

Table S1 :
Comparison of SERS detection performance of Ag-Au NPs film substrate with the previous reports.

Table S2 .
Comparison of the SERS-detection parameters for Ag-Au NPs films substrate with recently reported SERS substrates for nanoplastic detection