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In order to increase the figure-of-merit FOM of the sensor, we employed a phase-sensitive setup in the transmission based on common-path phase quadrature interferometry to evaluate the phase difference between the spectra measured along the axes of asymmetric NPs [ 52 ], [ 53 ], [ 54 ]. We also performed SERS analysis to verify the physical adsorption of the Thiram molecules on the substrate as a function of different concentrations. Although there have been many other attempts to detect Thiram using plasmonic sensors [ 55 ], [ 56 ], [ 57 ], [ 58 ], [ 59 ], [ 60 ], to the best of our knowledge, this is the first time that this is done using the LSPR approach and nanosensors fabricated with a top-down technique.

These results indicate that the LSPR nanosensors developed here are expected to demonstrate a wide range of applications for the detection of analytes of environmental and biological interest. Furthermore, our achievements open the way to use the ThMo pattern in dual-mode plasmonic sensors, combining LSPR and SERS analysis, making it more versatile for applications and more attractive from the cost-effectiveness ratio point of view.

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The side size d of NPs was, in all cases, nm. All solvents were purchased from Sigma Aldrich. In order to investigate both the near-field and the expected spectral response of the nano-arrays considered, we performed numerical calculations using the finite difference in time domain FDTD, home-made code method.

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In both calculations, we considered four order ThMo lattices with semiinfinite both the glass substrate BK7 and the air cover, whereas the thicknesses for the Au pillar and ITO layer used were, respectively, 50 nm and 15 nm. An incident laser wavelength of nm, polarized in the plane of the NPs, was used to stimulate the pattern. Perfectly matched layer PML boundary conditions on all directions were used.

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The electric component Ex intensity, parallel to the substrate surface, was calculated 20 nm above the plane of the NPs to avoid stair-stepped approximation error [ 62 ]. Calculated fields are quite regular in each nanopattern, and they are generated by both plasmonic and photonic coupling [ 63 ], [ 64 ].

Field properties are similar for square and circular NPs, with a distribution more uniform in the case of square NPs. In the case of triangular NPs, the near-field achieved seems to be much more localized between two close elements and it decreases rapidly moving away from them. In this last case, the simulations show the highest value for the hot-spot area. Spectral response in transmission was obtained by means of fast Fourier transform FFT of the response time. Optical properties of the fabricated plasmonic nanostructures have been evaluated via UV-vis adsorption spectroscopy based on intensity interrogation and SERS.

UV-vis extinction measurements allow to investigate the LSPR of the nanostructures, and they were realized using the setup shown in Figure 3 A. A Set up used to measure UV-vis extinction spectra. B Phase-sensitive setup based on common-path phase quadrature interferometry used for LOD evaluation of Thiram pesticide. N,N-dimethylcarbamodithioate Thiram is purchased from Merck. Working solutions were prepared by adequate dilution of the stock solution.

Chemisorption of Thiram on the Au nanostructured substrate is simply obtained by depositing on the sample a solution of Thiram for 12 h overnight. In Figure 5 D—F, we report the experimental trends of the LSPR peaks versus the refractive index of the medium achieved for the ThMo investigated with different pillar shape and a parameter. The trend is linear in each case, yielding a higher value of m when a increases. Calculated spectra are in good agreement with the experimental ones showing the same red shift and LSPR peaks near those found in the experiment.

We ascribe the differences between experimental and simulated spectra to the approximations made in the calculations, in particular, related to the smallest order of the pattern ThMo taken into account four orders necessary to reduce the computation time. At first glance, we can ascribe the higher m of square NPs arrays to their higher reference plasmon wavelength peak in air compared to those of circular and triangular ones, but a more in-depth analysis can be made by taking into account the numerical computations realized.

Trends of the LSPR peak wavelength versus the refractive index of the medium achieved for ThMo investigated with different minimum interparticle distance a and circular D , square E , and triangular F NPs.

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In B and E , the spectral response obtained by numerical simulations is reported in grey color. From the comparison between the near fields simulated Figure 2 and experimental results, it is worth noting that we found higher experimental m values corresponding with field distributions that appear more uniform and less localized.

In particular, FDTD simulations show clearly, as in regions far from the NPs where the near-field distribution is due mainly to the photonic coupling and to the multi-scattering process, that the field intensity for the three kinds of ThMo considered is higher in the case of NPs with square shape compared to those of the circular and triangular shapes. This achievement perfectly matches the trend of the experimentally measured sensitivity shown in Table 1. This agreement can be explained by referring to the unique optical interaction properties of aperiodic nanopattern.

Due to the increased structural disorder, the aperiodic arrays are strongly coupled in both the plasmonic near-field regime short-range coupling , which mainly determines the presence of hot-spot area between two near pillars, and the photonic diffractive one long-range coupling , which essentially affects the field distribution in nanopattern regions far from the particles [ 63 ]. In particular, the efficient photonic coupling in aperiodic arrays, associated with their high number of in-plane multi-scattering processes, enables both enhanced field states that are spatially distributed over larger areas and higher photon dwelling times compared to periodic patterns where scattered photons easily escape from the surface.

In fact, these characteristics improve the light-analyte interaction, enhancing the sensitivity of the system and making these types of patterns promising to develop advanced sensing devices. Therefore, we can assert that the higher m of square NPs array should arise from the better long-range coupling and higher number of light scattering process enabled from this geometry compared to the ones possible for ThMo arrays made of circular or triangular NPs.

In order to confirm this thesis and highlight the impact that the long-range coupling can have on the sensor performance, we characterized a periodic pattern based on Au nanoprisms with size comparable to those of the ThMo arrays. This gap on the sensitivity found in the comparison of the two kinds of nanostructures investigated periodic and aperiodic provides further support to the high impact that the higher number of multi-scattering process in a pattern with greater degree of disorder can have in the realization of sensors with a detection more accurate and sensitive.

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Such results show how the nanopatterns considered are well suited as LSPR sensor for detection application of analytes through the use of a low-cost experimental setup. As the results of the characterization in terms of sensitivity m and linearity were promising, we used this approach to investigate an analyte of environmental interest. Nevertheless, the values of FOM found for all nanopatterns analyzed, ranging from 0. In this regard, we further investigated the sensing performance of the ThMo arrays in the detection of the Thiram pesticide by the use of a common-path phase quadrature interferometry setup, described in the references [ 52 ], [ 53 ], in a transmission configuration see Figure 3 B. Unlike the case of symmetrical NPs square or circular , for triangular NPs, due to their asymmetry, the transmission spectra measured for the s and p polarizations are shifted [ 54 ]. Here, LOD is defined as the minimum detectable refractive index change due to the presence of the pesticide. Chemisorption of Thiram on the gold nano-structured surface of our sensor is facilitated by the presence of sulfur in the molecular structure of the compound. The presence of sulfur in the thioamide groups of a disulfide-based molecule surely allows its adsorption on the Au surface.

The signal peaks show a good signal-to-noise ratio, and their FWHMs are found to vary between 1. It is worth noting how the phase-sensitive measurements performed allow signals with line widths about times narrower compared to their intensity-based counterparts. In Figure 6 C, the signal wavelength shifts are plotted as a function of the Thiram concentrations.

Each data point is the average value of measures relative to four different sensors analyzed with identical concentrations. Furthermore, we used Thiram as molecular analyte to evaluate the SERS activity of the same ThMo array and to verify the physical adsorption of the substrate as a function of different concentrations of the Thiram molecules. In summary, we have extensively analyzed the characteristics of the LSPR of engineered aperiodic nanostructures with the arrangement in ThMo NPs as a function of both their shapes and interparticle distances, comparing experimental and numerical results.

Our results have shown that FOM in based refractive index sensing can be largely increased by measuring the phase difference of the transmitted beam instead of its intensity only. We used our nanosensors for the detection of Thiram pesticide, and to the best of our knowledge, this is the first time that this is done using the LSPR approach and devices fabricated with a top-down technique.

In conclusion, this work presents significant developments in the use of LSPR sensors based on aperiodic nanopatterns for the detection of pesticides in samples of water. Our results open the possibilities to engineer low-cost portable sensors for the detection of biological and environmental analytes, sensitively, rapidly, and in low volume samples. We thank Dr. Molecular plasmonics for nanoscale spectroscopy. Chem Soc Rev ;— Plasmonics: theory and applications. Recent developments and future directions in SERS for bioanalysis. Phys Chem Chem Phys ;— Surface-enhanced Raman scattering in cancer detection and imaging.

Trends Biotechnol ;— Xie W, Schlucker S. Medical applications of surface-enhanced Raman scattering.

Table of Contents

Aroca RF. Plasmon enhanced spectroscopy. Biosensing with plasmonic nanosensors. Nat Mater ;— Recent progress in surface enhanced Raman spectroscopy for the detection of environmental pollutants. Microchim Acta ;— A plasmonic nanostructure fabricated by electron beam lithography as a sensitive and highly homogeneous SERS substrate for bio-sensing applications.

Vib Spectro ;— Nanomaterials enhanced surface plasmon resonance for biological and chemical sensing applications. Plasmonicnanoprobes: from chemical sensing to medical diagnostics and therapy. Nanoscale ;— Zheng J, He L. Surface-enhanced Raman spectroscopy for the chemical analysis of food. Fang Z, Zhu X. Plasmonics in nanostructures. Adv Mater ;— Ultra sensitive label free surface enhanced Ramanspectroscopy method for the detection of biomolecules. Talanta ;—5. Nanofabricated SERS-active substrates for single-molecule to virus detection in vitro: a review. Biosens Bioelectron ;— Localized surface plasmon resonance biosensing: current challenges and approaches.