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Coupling of Plasmonic Hot Spots with Shurikens for Superchiral SERS-based Enantiomer Recognition

Detection of enantiomers is a challenging problem in drug development as well as environmental and food quality monitoring where traditional optical detection methods suffer from low signals and sensitivity. Application of surface enhanced Raman scattering (SERS) for enantiomeric discrimination is a powerful approach for the analysis of optically active small organic or large biomolecules. In this work, for enantio-selective sensing we proposed the coupling of disposable chiral plasmonic shurikens supporting the chiral near-field distribution with SERS active silver nanoclusters. As a result of the plasmonic coupling significant difference in SERS response of optically active analytes is observed. This is hypothesized and confirmed by numerical simulations to be a consequence of the silver particles being excited by superchiral fields generated at the surface or the probe molecules achieving additional polarization. The plasmon coupling phenomena was found to be extremely sensitive to slight variations in shuriken geometry, silver nanostructured layer parameters, and SERS excitation wavelength(s). Designed structures were able to discriminate cysteine enantiomers at concentrations in the nanomolar range and probe biomolecular chirality, using a common Raman spectrometer within several minutes. The combination of disposable plasmonic substrates with specific near-field polarization can make the SERS enantiomer discrimination a commonly available technique using standard Raman spectrometers.


Introduction
Utilization of Raman measurement-based enantiomer analysis, for example, Raman optical activity (ROA) measurements, are a useful tool in the field of chiral identification of drugs or 3D conformation of biomolecules. [1][2][3][4][5] However, the ROA signal is intrinsically weak and should be amplified. The plasmonic enhancement on a surface of noble metal nanostructures was proposed to provide surface-enhanced Raman optical activity (SEROA) signals. 6 In SEROA, the enantiomer probe is deposited on the plasmon active surface, where the excitation of plasmonic hot spots leads to high enhancement of Raman scattering. 7-10 SEROA measurements can be performed in two experimental arrangements, either using a circularly polarized excitation beam or by detecting circularly polarized photons of the scattered light. However, the plasmonic support plays a key role in both arrangements to ensure the enhanced interaction of light with the probed matter. 7,11 However, SEROA is impaired by the weak dependence of local polarization in the plasmonic dipole (in general case, in an almost arbitrary configuration of plasmon active nanoparticles or hot spots) on the initial light polarization, and commonly this dependence can be considered as linear. [11][12][13] Thus, the enantiomers placed in hot spots are subjected mainly to noncircular polarization of the near plasmonic field and the resulting contribution from chirality in scattered light is thus smallscale. 11 Indeed, this fact does not prevent the utilization of SEROA but forced most researchers to find minuscule differences in circularly polarized scattered light against the dominant background of non-chiral photons. 7,14,15 On the other hand, the local plasmon polarization can be tuned by the construction of more sophisticated plasmon-active structures than nanoparticle dimers or multimers. [16][17][18][19][20][21] The specific shape of plasmonic nanostructures will induce the circularly polarized plasmon field to trigger enantiomers and produce different Raman responses. 17,18 In addition, the plasmon polarization excited on these structures can be principally different from the incident light polarization with the appearance of a superchiral plasmonic near field, which potentially allows the SEROA measurement with the utilization of common Raman spectrometers. [22][23][24][25] In this work, we propose the coupling of chiral shuriken-shaped plasmonic structures with plasmonic hot spots, originating from deposited silver clusters for SERS-based enantiomer recognition. 24,26 SERS measurements indicate the chiral-dependent intensity of the scattered light from enantiomer probes, where the SERS intensity was determined by the handedness of the shuriken and the probe. The analytical signal was achieved using a common Raman spectrometer without the introduction of a circularly polarized excitation laser or a polarization-sensitive detector. 3

Design of optically and SERS-active structures.
The nano-structures for SERS-based enantiomer discrimination are schematically presented in Fig.   1a. As a chiral support, we used the previously developed nanostructures with a specific shape -gold shurikens (SEM images with typical morphology in Fig. 1b). [22][23][24][25][26][27] The shuriken arrays were prepared using injection molding were designed with a six-fold rotation symmetry of either left-handed (LH) or right-handed (RH) symmetry with slight differences in the central diameter and thickness of shuriken arms (see Figs. S1-S3 and related discussion). Coupling of magnetic and electric modes excited across the illuminated shuriken structures, superchiral near fields that display greater chiral asymmetry than the circularly polarized plane waves, are generated at the surface (detailed description is given in previous works). [28][29][30] In particular, the shurikens generate fields with large chiral hotspots and overall net chirality at the metal-dielectric boundary (Figs. S4, S5). Indeed, different interaction of handedness of excited superchiral fields with handedness of surrounding enantiomer (for example, deposited as an additional layer) allows their enantioselective sensing, as was demonstrated previously. 27,[31][32][33][34] In particular, shuriken structures were previously employed for protein secondary structure sensing through the measurement of the shifts of the resonance wavelength position or light phase retardation. 22,35 However, our initial attempts to utilize pristine gold shurikens for SERS-based enantiomer discrimination failed by the fact that as-prepared substrates have shown insignificant SERS enhancement. To introduce additional plasmonic hot spots, we sputtered an additional imperfect silver layer with a cluster-like nature (Fig. S6-S10 and related discussions). In this case, the SERS enhancement can be expected due to the rough nature of the deposited film. 36,37 The deposited silver clusters can be approximated as an array of plasmonic dipoles, which commonly ensure the high degree of SERS enhancement. The final structure (Fig. 1a) represents the Au shuriken coupled with Ag nanoclusters, hence, producing a surface of randomly oriented plasmonic dipoles exposed to the superchiral optical near fields generated by the plasmonic shurikens. Furthermore, created samples are referred to as LH-or RH-Sx/Ag, where LH-or RHdescribes the shuriken left or right-handed rotation symmetry, Sx indicates the shuriken geometry (see SI for details) and Ag highlights the presence of added silver nanoclusters.

Optimization of LH-or RH-Sx/Ag structures.
SERS efficiency of the created Sx/Ag structures was firstly evaluated using the achiral model SERS probe -R6G (Figs. 1c, d, and Fig. S11). Two basic parameters were considered: (i) the structure of SERS-responsive silver layer ("apparent" thickness and roughness, adjusted by silver deposition time) and (ii) the shape of the shurikens. We used 780 nm as the Raman excitation wavelength and 4 the illumination of only Au shurikens does not produce any intensive SERS signal ( Fig. 1c bottom graph) as the Au only shuriken nanostructures most likely produces weak plasmonic energy concentrations since the incident photon is "smeared" across the shuriken structure (Fig. S5), with a size comparable to the excitation wavelength. In other words, the incident photons can be absorbed even in this case, but the evanescent field of the excited chiral plasmon is restricted by the relatively large space (determined by each shuriken unit geometry). This disagrees with the common plasmonicbased SERS requirement, where the photon energy should be focused in a plasmonic hot spot with 1-2 nm dimension. In turn, silver deposition led to the generation of plasmon active silver clusters, which provide the plasmonic hot spots desired for SERS. After silver deposition, we observed the appearance of intensive SERS response from R6G molecules, which have a similar trend on both kinds of shurikens rotations (Fig. 1c, d and Fig. S11). Since the Ag cluster geometry and gaps between clusters are determined by the deposition time, there is an increase in R6G intensity with deposition time (up to 1.3±0.8 nm of "effective" thickness) followed by the decrease with the prolongation of deposition time due to clusters merging at a later stage. This tendency was observed for all shuriken geometries (S1-S6), but was especially pronounced in LH-S3/Ag or RH-S3/Ag cases. The most enhanced response was in case of 1.3 nm Ag and was therefore chosen as the optimal "averaged" thickness. In addition to deposition time, shuriken geometry significantly affects the absolute SERS efficiency for all thicknesses (Fig. 1 c, d). In particular, the SERS enhancement factor (EF) on S5/Ag and S6/Ag was less than 1200; it was moderate on S4/Ag and S2/Ag shuriken types (2247 and 3250); and the highest EF=3852 in the case of S3/Ag structures. Hence, we conclude that the plasmon coupling between the shuriken and silver hot spots does occur and is determined by plasmonic modes excited on different shurikens types. Such a difference in SERS efficiency, appearing due to a slight change in shuriken geometry (Figs. S1-S4 and related discussion), indicates indirectly that the plasmonic coupling may have a resonant nature.
The impact of Ag deposition on shurikens (RH/LH-S1-6/Ag) optical response is illustrated in Fig. S4. The effect is manifested by the optical reflectivity spectrum as a function of 1.3 nm Ag layer.
The changes in reflectivity after Ag deposition were hardly pronounced for S1/Ag, S2/Ag, and S4/Ag structures as well as for S5/Ag and S6/Ag structures. It should be also noted that the pristine S5 and S6 shurikens structure show less intensive extinction and ORD peaks. Thus, the lower efficiency of these structures in enantioselective discrimination can be expected. In turn, the S3/Ag samples show increased absorption, which will potentially lead to increase in the field strengths generated. Even at these wavelengths, the measurement of optical rotation density (ORD) indicates the appearance of ORD bands with opposite signs and mirror-like symmetry for LH-and RH-samples (Fig. S4b).
Hence, the deposition of Ag layer led to the shift in the reflection (absorption) spectra of the shurikens arrays, especially pronounced for S3/Ag samples. The coupling of two plasmon active nanostructures commonly results in the changes of their plasmon absorption bands and optical response. 38 We observe a similar effect, more noticeable for S3/Ag samples, which also corresponds well with the presence of some plasmonic coupling between shuriken units and Ag clusters array. Considering the SERS, reflectance and ORD measurements results, the (LH/RH)-S3/Ag (1.3 nm) structures were chosen for all further enantioselective SERS measurements.
It should be also noted that the applied SERS excitation wavelength 780 nm is close to Sx/Ag resonance and is far away from the common wavelengths (near 500 nm) used for SERS measurements on silver nanostructured layers. Indeed, deposition of R6G molecules on Ag (1.3 nm)/Si and further SERS measurements with 780 nm excitation did not produce any significant SERS response ( Fig.   S12 and related discussion). Therefore, we can conclude that Ag hot spots are primarily excited by shuriken near field, but not by the incident photons. The silver dipoles are exposed to the superchiral field with enhanced chiral asymmetry, which can potentially result in a higher contribution of circular polarization in the local electric field. As a result, a significant difference in the Raman scattering spectra of small (cysteine) or large (DNA) biomolecular enantiomers could be expected in this case.

SERS-based discrimination of small molecules -L/D cysteine case.
Challenging above assumption, we deposited L/D-cysteine on LH-or RH S3/Ag structures for further SERS measurements. Considering that only molecules close to plasmonic hot spots can produce chiral SERS signal, we spin-coated cysteine solutions to generate thin films using varying cysteine concentrations to check the effect of probe analyte quantities on the plasmonic surface.
Conditions of spin-coating were chosen to form a homogeneous, amorphous layer of cysteine (Fig. S13-S16 and related discussions). Results of L-or D-cysteine enantiomers measurements performed in common SERS mode are presented in Fig. 2. The SERS spectrum is identical to the previously published for cysteine in the vicinity of Ag coated Au nanoparticles (comparative results are given in Tab. S1 [37][38][39][40][41][42][43] ). The slight shift of peak position is likely due to the different excitation wavelengths and the interaction of plasmonic substrate with the cysteine via sulphur, amino and carboxylic acid groups. [41][42][43] In our case, however, the intensities of measured spectra are significantly affected by the enantiomers and shuriken handedness (Fig. 2b,c), RH-or LH-S3/Ag. The pronounced SERS bands were observed in the case of L-cysteine deposited on RH-S3/Ag structures, unlike the case of Dcysteine enantiomers deposited on the same substrate. On the other hand, D-cysteine enantiomer SERS spectra are rather intense in the case of LH-S3/Ag plasmonic substrate (Fig. S17). The significant differences in characteristic SERS bands intensity (at 1044 cm -1 ) are observed from 10 -8 M up to the 10 -4 M initial concentration of cysteine enantiomers. Higher cysteine concentrations led 6 to an overall decrease of SERS efficiency without the ability to distinguish L-or D-cysteine enantiomers (Fig. S18). Generally, the observed dependence of L/D-cysteine SERS intensity on handedness of plasmonic substrate clearly indicate that the coupling of Au shurikens and SERS active Ag layer is applicable for enantiomers discrimination up to 10 -8 M using standard Raman spectrometer.
To further verify the observed difference in enantioselective excitation by coupled chiral plasmonic structures, we investigated the homogeneity of intensities for characteristic cysteinerelated peak at 1044 cm -1 on LH-and RH-S3/Ag structures (Fig. 3). In this case we observed the appropriate level of D-cysteine SERS response which is higher by ≈ 5 times on LH-S3/Ag substrate compared to RH-S3/Ag (in latter case the inhomogeneous region on the right map rather refers to background deviations). Despite the fact that cysteine is deposited over the entire surface of LH-and RH-S3/Ag samples, there is no SERS effect on the residual parts of the initially flat Au film (Fig.   3b). Thus, dramatic difference in SERS mapping intensities further confirms the crucial role of plasmonic substrate chiral pattering. Specifically designed Au shuriken -Ag nanocluster substrate enable to perform SEROA-like measurements with a common Raman spectrometer with reliable sensitivity.

Impact of SERS excitation wavelength.
To reveal the excitation site of coupled SERS structure and impact of shurikens rotation (and corresponding optical rotation signs), we estimated the impact of SERS excitation wavelengths on chiral SERS efficiency. The pristine S3 shurikens show an apparent plasmon resonance band near 750 nm, which is slightly enhanced and broadened after Ag layer deposition (Fig. S4). In turn, ORD dependencies, presented in Fig. S4b show the less intensive ORD peaks of S5 and S6 shuriken structures than the other structures. This difference in ORD can explain the poorer enantioselective SERS performance on LH-and RH-S5,6/Ag substrates. In addition, the chiral response is also observed near 600 nm. Hence, utilization of standard Raman laser wavelengths near 600 and 750 nm could also result in the appearance of chiral response from the targeted analyte. In turn, in spectral regions of two other common Raman excitation wavelengths (530 and 850 nm), the ORD signal is close to zero. (Fig. S4 and S19).
Impacts of SERS excitation wavelength on cysteine enantiomeric SERS response are presented in Fig. S20 and S21. For the sake of clarity, Fig. S22 shows a relative difference in the intensities of the SERS response of the RH-S3/Ag and LH-S3/Ag, plotted as "IRH-S3/Ag -ILH-S3/Ag" for L-cysteine using different excitation wavelengths. The excitation at 780 nm wavelength displays an apparent difference in L-cysteine SERS response as a function of shuriken rotation (Fig. S22). In contrast, utilization of 850 and 530 nm did not lead to the difference between spectra on both kinds of substrates (LH-S3/Ag vs. RH-S3/Ag). More interestingly, the 630 nm excitation wavelength also produces a differencethe chiral response is flipped compared to the SERS response at 780 nm excitation. The observed results are consistent with the optical rotation for LH and RH shurikens structures (Fig. S4). When optical rotation is positive (780 nm -Fig. S19), L-cysteine produces enhanced SERS signal on RH-S3/Ag substrates and vice versa. SERS signal from D-cysteine is more pronounced from LH-S3/Ag in the case of the negative value of optical rotation (630 nm -Fig. S19).
Hence, this confirms that SERS spectra of enantiomer(s) measured at different wavelengths, corresponding to opposite signs of shuriken optical rotation, will invert the selective chiral SERS response. This dependence establishes the link between enantiomeric selectivity and excitation wavelength and provides an additional degree of control in enantiomer recognition reliability. It should also be noted that the apparent difference in L-cysteine SERS response was reached solely with wavelengths corresponding to the shuriken plasmon absorption (but not with commonly used for rough Ag surface excitation wavelength -530 nm). This difference confirms our previous assumption that Ag hot spot excitation by shuriken superchiral near field plays a crucial role in SERSbased enantiomeric discrimination.

Enantiomers mixture analysis and substrates re-utilization.
Commonly analytical probes contain both enantiomers, where the enantioselective sensing become more challenging. Taking advantages of coupled shuriken Ag hot spots substrate, the chiral detection performance of RH-and LH-S3/Ag substrates was verified by the SERS analysis of racemic mixtures containing 10 -5 M L-cysteine and 10 -6 M D-cysteine or vice versa. The measured averaged peak intensities were compared to previously "found" enantiomers concentrations using equations (Fig. 2b,c) Int1044=702×log(c)+5741.9 for L-cysteine and Int1044=686.6×log(c)+5603 for D-cysteine ( Fig. 3c, 4). "Found" concentration values converged with the added amount of L/D-cysteine within the experimental error. As is evident, developed SERS structure determines enantiomer concentration within ~98% accuracy even in case of minor concentration of target enantiomer. So, the proposed approach allows the identification of even negligible enantiomer presence without preliminary racemic mixture separation or chiral extraction.
Further, we addressed the possibility of regeneration of the RH-or LH-S3/Ag substrates after chiral sensing of L/D-cysteine, which is important particularity for various applications. 44 Firstly, we performed a few cycles of substrates utilization/regeneration within one day or several days (in the last case, substrates were stored in air or vacuum). Obtained results, presented in Figs. S23 and S24 show that the characteristic cysteine SERS bands fully disappear after the substrate regeneration by 8 gentle ultrasonication in methanol. 45 Several subsequent cycles of substrate utilization/regeneration reveal that the substrates can be re-used for up to 5 cycles without significant loss of functionality. In case of longer storage between regeneration/utilization, the more pronounced decrease of the intensity of cysteine-related Raman peaks was observed (probably due to Ag oxidation on air, evident as the appearance of dark spots on samples surface). So, freshly prepared RH/LH-S3/Ag can be re-used multiple times for chiral SERS sensing and with intermediate storage under vacuum.

SERS-based discrimination of large biomolecules -DNA case.
Utilizing chiral SERS for analysis of large biomolecules has fuelled our interest to apply RH/LH-S3/Ag for DNA analysis (DNA structure is in Fig. 5). We used double stranded DNA (this type of biomolecules commonly shows the circular dichroism response in UV range) 47 consisting of 20 bases with the structure close to an ideal helical geometry with ca 2 turns. We verified the possible increase of substrate RH/LH-S3/Ag temperature during SERS measurements, which can potentially cause innate denaturation of DNA. For this purpose, we grafted RH/LH-S3/Ag substrates with -C6H4-C≡N chemical moieties, which served as a molecular temperature probe ( Fig. S25 and related discussion). 48,49 Comparison of measured SERS spectra collected at RT and elevated temperatures (external heating) indicated the insignificant increase of local substrate temperature under Raman 9 laser illumination. Results of SERS sensing of DNA probe on LH-or RH-S3/Ag samples are presented in Fig. 5 (peak affiliation is given in Table S2). The apparent spectral difference, significant SERS bands enhancement as well as their spectral separation was observed solely in the case of LH-S3/Au substrate, while the application of RH-S3/Au substrate did not produce any significant SERS signal. To sum up, the proposed approach can be used for the analysis of both small organic molecules and large biomolecules. In constant to L/D-cysteine, where amino and carboxy-related Raman bands are enhanced, the surface enhancement of DNA SERS bands occurs homogeneously. This fact indirectly indicates that the molecular response can also be affected by the shuriken-induced polarization of the probe biomolecules, which further interact with plasmonic hot spots excited on silver clusters.

Proposed mechanism of enantio-selective SERS enhancement.
Our results demonstrate coupling of Ag-derived hot spots with chiral shuriken structures that produces an effective chiral SERS signal for enantiomer discrimination using a common Raman spectrometer. Based on the experimental results we propose several possible mechanisms. The first, one is the coupling of Ag dipoles and shurikens chiral structures, where silver hot spots are excited by the shuriken superchiral near field. In this case, the transition of superchirality from the shuriken to Ag hot spot can be expected. As a result, the probe molecules will be excited by a higher degree of local circular electric field polarization, which potentially ensures more resonant differences in SERS spectra of the probe enantiomers. Liu et al. proposed a similar concept using chiral Au nanofibers and the chiral shuriken design leads to large chiral near fields (Fig. S4), unlike simpler dimers or nanorods, justifying the possibility of having a strong impact on the polarized excitation of the Ag dipoles. 17 We also perform an additional simulation of the behavior of Ag nanostructure (estimated as a single nanoparticle or two nanoparticles) subjected to shuriken plasmon nearfields.
Simulation results (Figs. S26, S27, and related discussion) indicate that under certain resonance conditions, the nanoparticle(s) is surrounded by relatively large chiral fields, which strongly modulate the behavior of the intrinsically achiral nanoparticles. Moreover, the excited near nanoparticle field is likely to be chiral in nature themselves and would affect the SERS response from the chiral molecules (taking into account the fact that even Ag nanostructures are the source of SERS). So, a difference in SERS efficiency (for various structures) and additional simulation results indicate that the plasmonic coupling may have a resonant nature which produces a localized superchiral field.
The second possibility is the overall enhancement of enantiomer spectra. This is determined solely by shuriken handedness and is independent on the probe molecule scattering tensor, concerning the position of optically active centers in the probe molecule structure. From this point of view, we can propose that the shuriken induces an overall polarization of analytes, which increases their Raman scattering coefficient in a similar way as it occurs in the case of metal-free SERS. Additional molecular polarization can provide the overall SERS intensity increase by a factor of 10-10 2 , which is in relatively good coincidence with the observed experimental data. 50 Still, the third explanation may consist in plasmon chirality transfer, occurring after the coupling of intrinsically non-chiral plasmon active nanoparticles (in our case -Ag clusters) and the chiral surrounding dielectric medium. [51][52][53][54][55] It could be expected that in our case (with the coupling of non-chiral plasmon active nanostructures and superchiral shuriken field) this phenomenon will be significantly enhanced and can ensure the increase of the enantioselective interaction of probe molecules with light, as was reported for common circular dichroism measurements. A fourth possible explanation is that the electric nearfield of the shuriken are affected by the chiral dielectric, effectively modulating the field intensity exciting the Ag clusters. The intensity and hence the SERS enhancement therefore become dependent on the enantiomeric properties of the dielectric. However, to achieve a change in the electric field that is significant enough to cause the results demonstrated would require cysteine to have chirality factor an order higher than considered practical.
At the moment, it is not possible to differentiate the exact mechanism for the observed enantioselective differences in SERS spectra. Moreover, the mechanisms described above can contribute simultaneously and synergistically. Further theoretical and experimental research in the field will be performed to elucidate the proper mechanism of the observed phenomena. However, it is clear that the proposed plasmon-active structures enable enantioselective SERS measurements using standard and inexpensive Raman spectrometers without the need of more complex ROA equipment. Moreover, the plasmon active substrates, including the shuriken support, are produced through scalable technology that can be considered as replaceable and disposable. 27 Hence, it will be possible to perform switching in Raman spectroscopy from normal to enantio-selective measurement modes by exchanging the disposable plasmonic substrates.

Conclusion
In this work, we demonstrated the key role of nanoarchitecture of plasmonic substrates in chiral SERS discrimination of small organic enantiomers and large biomolecules. The proposed design of chiral and SERS active substrates is based on the coupling of chiral plasmonic shurikens with array of Ag clusters. As a result, the Ag plasmonic clusters multimers are exposed to the superchiral field close to shurikens, followed by expected excitation of circular polarization in plasmonic silver hot spots. This combination allows us to observe the high differences in the SERS intensities of chiral probed molecules (L/D-cysteine and DNA molecules) as a function of the chirality of shuriken or molecule. Optimization of this enantioselective detection/recognition was performed by varying (i) Ag layer thickness/roughness, (ii) geometry of shuriken, and (iii) selection of excitation wavelength, which should correspond to the chiral response of the coupled structure.
These dependencies indicate the strong (and probably resonant) plasmonic coupling, which allows us to perform surface-enhanced, enantio-selective measurements with the wavelength-dependent enhancement of different enantiomers. Moreover, all measurements were performed in several minutes using a common Raman spectrometer and are also used to determine the enantiomeric composition in racemic mixtures. We demonstrated that a specific design of the plasmonic substrate with appropriately selected excitation wavelength could ensure the generation of nonlinearly polarized electric fields inside plasmonic hot spots triggering and enantiospecific SERS response from chiral molecules. This was experimentally confirmed by the dependency of the chiral selective SERS response on the excitation wavelength and the chiral response of the shurikens, supported by numerical simulations. The developed plasmon-active substrate can ensure the SEROA-like measurements using a common Raman spectrometer without the necessity of a chiral light source or chiral sensitive detectors. Such a substrate and methodology are promising for rapid and simplified enantiomeric discrimination in pharmaceutical and chemical industrial applications.

Experimental Section
Preparation and characterization of Sx/Ag structures are described in SI. Briefly, the templateassisted technique was used for the creation of Au Shurikens, followed by vacuum sputtering of silver.
Surface morphology and metals distribution were checked using AFM and SEM-EDX measurements.
Polarization-dependent reflectivity was determined using homemade equipment (see SI for detail).
Raman spectra were recorded using a ProRaman-L and inVia spectrometers (530, 630, 780, and 850 nm excitation wavelengths). Spectra data collection was performed at the same experimental conditions (laser power, collection time), optimized for each used wavelength. From each sample, 10 random points were analysed on each shuriken array and all spectra were baseline corrected and smoothed using an 11-point averaging smoothing algorithm to reduce the baseline variability in the region between 450 -2000 cm −1 , using Omnic Professional Software Suite (Thermo Scientific, Inc., Madison, WI). Deposition and preparation of probe analytes (cysteine enantiomers and DNA molecules) are also described in SI. Simulations were performed using COMSOL v5.6, a software for simulating physics using finite element method. Periodic boundary conditions were used to emulate the array of nanostructures. Perfectly matched layer conditions were used above and below the input and output ports. Linearly polarized EM wave was applied at normal incidence onto the structure.

Acknowledgment
This work was financially supported by GACR under the project 20-19353S (OL) and Tomsk Polytechnic University Enhancement program (OG). AK would like to acknowledge support by the UKRI, EPSRC (EP/S001514/1 and EP/S029168/1) and the James Watt Nanofabrication Centre. NG and MK acknowledge support from EPSRC (EP/S012745/1 and EP/S029168/1). We would also like to acknowledge Prof. Laurence Barron for his academic discussions and support.