Applications of Shell-Isolated Nanoparticle-Enhanced Raman Spectroscopy

: The surface-enhanced Raman scattering (SERS) is mainly used as an analysis or detection tool of biological and chemical molecules. Since the last decade, an alternative branch of the SERS effect has been explored, and named shell-isolated nanoparticle Raman spectroscopy (SHINERS) which was discovered in 2010. In SHINERS, plasmonic cores are used for enhancing the Raman signal of molecules, and a very thin shell of silica is generally employed for improving the thermal and chemical stability of plasmonic cores that is of great interest in the speciﬁc case of catalytic reactions under difﬁcult conditions. Moreover, thanks to its great surface sensitivity, SHINERS can enable the investigation at liquid–solid interfaces. In last two years (2019–2020), recent insights in this alternative SERS ﬁeld were reported. Thus, this mini-review is centered on the applications of shell-isolated nanoparticle Raman spectroscopy to the reactions with CO molecules, other surface catalytic reactions, and the detection of molecules and ions.

The goal of this mini-review is to present the recent advances on the most used applications of SHINERS, such as the catalytic reaction-monitoring processes and the detection of molecules, over the period 2019-2020. Firstly, we will explore the SHINERS applications to the reactions with CO molecules, which are well-known model reactions, then other surface catalytic reactions, and finally the detection of molecules and ions in order to examine the potential of this SHINERS technique. In the final section, we will discuss points to be improved and advantages of the SHINERS technique, and we will address the future directions of this latter.

What Is Shell-Isolated Nanoparticle-Enhanced Raman Spectroscopy?
The shell-isolated nanoparticle-enhanced Raman spectroscopy belongs to the SERS field, and is based on the enhancement of the Raman signal obtained with strong electric fields coming from plasmonic core-shell nanoparticles. In SHINERS, each core-shell nanoparticle plays the role of a metallic tip as for the tip-enhanced Raman spectroscopy (TERS), and this technique allows to obtain a couple of thousand of "TERS tips" on the substrate surface to be analyzed. Thus, the enhanced Raman signal can be jointly obtained from all these plasmonic core-shell nanoparticles ("TERS tips"), allowing a gain of two to three magnitude orders compared to a single TERS tip. Furthermore, the use of the metallic nanoparticles coated with a chemically inert shell can enable the protection of the plasmonic core (SERS-active part) from the substrate surface to be analyzed and the environment. This inert shell can conform to different morphologies of substrates, and also prevent the agglomeration of these core-shell nanoparticles and the oxidation of their plasmonic core. The principal merits of such a technique are a more significant detection sensitivity and a great number of practical applications in life and materials sciences, as well as in food science and environmental pollution.

SHINERS Application to the Reactions with CO Molecules
In this section, we present a couple of investigations on the reactions with CO molecules (see Table 1) [82][83][84][85][86]. These reactions with CO molecules are well-known model reactions.  Figure 1a) [82]. The authors demonstrated that PtFe catalysts were more active and stable than Pt ones in the CO oxidation. In the CO oxidation, two Raman peaks have been observed at 397 and 485 cm −1 for Pt catalysts and at 389 and 480 cm −1 for PtFe catalysts, both corresponding to the adsorption of Pt-CO (see Figure 1b). The redshift of these two Raman peaks recorded for the case with PtFe catalysts has indicated that the CO adsorption was lower on PtFe than on Pt (see the inset of Figure 1b). Moreover, three Raman peaks of oxygen species were detected with PtFe nanocatalysts, indicating that the Pt-C binding was weakened by the presence of the ferrous center thus producing oxygen species (see Figure 1b) [82].  (110) and Pt (111)) with the SHIN-ERS technique by using Au@SiO 2 nanoparticles [83]. The authors have observed that the activity of CO electrooxidation was higher for Pt(111)/Pt(100) surfaces than Pt (110) surface. This increased activity of CO electrooxidation for Pt(111)/Pt(100) surfaces was due to the presence of OH and COOH species. For Pt (110) surface, this activity was weaker due to its high adsorption and coverage of CO on this surface [83]. Furthermore, Wondergem et al. have demonstrated the CO adsorption on Pt nanoparticles which are themselves deposited on Au@SiO 2 nanoparticles (see Figure 2a,b) [84]. This investigation was realized by employing the SHINERS technique. From the SHINERS spectra, two characteristic Raman peaks of CO adsorption were recorded in the two ranges of wavenumbers: 350-600 cm −1 and 1900-2150 cm −1 . The first Raman peak corresponds to the CO adsorbed on Pt in a bridge configuration located at 430 and 2010 cm −1 in these two ranges of wavenumbers (see Figure 2c,d), then the second one corresponds to the CO adsorbed on Pt in a linear configuration located at 505 and 2070 cm −1 in these two wavenumber ranges (see Figure 2c,d) [84]. Next, the same group has studied the effect of the fabrication method of nickel (Ni) catalysts on Au@SiO 2 nanoparticles for SHINERS investigations [85]. Three methods have been tested: spark ablation (SA), colloidal deposition (Col) and precusor (Pr) method. The authors have studied the CO adsorption on these three types of Ni catalysts, and concluded that Pr and Col methods were not suitable for the SHINERS technique due to the use of a high-temperature treatment of reduction. Finally, the SA technique is the most efficient for direct deposition of the nickel catalyst on Au@SiO 2 nanoparticles [85]. Hartman et al. have investigated the support effect on the interaction of rhodium (Rh) with CO molecules probed by SHINERS technique [86]. Two types of extrudate support have been tested by introducing alternatively CO and H 2 : the first one was Rh/SiO 2 on which were deposited Au@SiO 2 nanoparticles and the second one was Rh/TiO 2 on which were also deposited Au@TiO 2 nanoparticles. Under the same conditions of CO then H 2 , the shifts of a Raman peak (named "unknown" by the authors) for the CO hydrogenation were of 70 cm −1 and 25 cm −1 for Rh/TiO 2 and Rh/SiO 2 extrudate supports, respectively. From these Raman shifts for the CO hydrogenation, the authors have deduced that the Rh/TiO 2 extrudate support had the strongest interaction with CO molecules during the catalytic process compared to the Rh/SiO 2 extrudate support. Thus, this powerful interaction has resulted in a catalyst with higher efficiency for the CO dissociation [86]. Zhang et al., have reported on the adsorption of CO molecules on Pt nanocatalysts, which were deposited on novel core-shell nanoparticles composed of a gold core covered by graphene layers (Au@G nanoparticles). The authors have demonstrated via SHINERS measurements that the adsorption of CO molecules on these Pt/Au@G nanoparticles has occurred in a linear configuration [87]. To finish this section, two groups have reported on the CO reduction catalysis on Cu foil by using the SHINERS technique [88,89]. The authors have employed Au@SiO 2 nanoparticles, and allowed them to deduce that the CuO x /(OH) y species were detected on the Cu foil during the CO reduction. Thus, the authors have concluded that the oxygenated species of Cu were unlikely to be the active sites easing the formation of C 2+ oxygenates during the process of CO reduction [88,89].

Samples
Core  (111)) was due to the more important presence of the primary alcohol group [90]. In the next two examples, the studies of the water oxidation and the configuration of the interfacial water are addressed. Firstly, Saeed et al., have reported on the employment of the SHINERS technique to analyze in realtime the mechanisms of water oxidation with iridium oxides (IrO x ) as electrocatalyst [91]. To do that, Au@SiO 2 nanoparticles were used and deposited on IrO x surface for the Raman characterization (SHINERS). Thus, the authors demonstrated that SHINERS allowed to observe in real-time the chemical changes on the IrO x surface during the oxidation of water [91]. Secondly, Li et al., have reported on the configuration of the interfacial water at Au(111) surface, probed by the SHINERS technique using Au@SiO 2 nanoparticles for this study [92]. The authors have observed redshifts of the Raman peak corresponding to the O − H stretching mode of the interfacial water when the potential went towards more negative values, indicating a configuration variation of interfacial water. Thus, the authors have shown three configurations of the interfacial water named parallel, one-H-down and two-H-down, respectively, when the potential shifted to more negative values [92]. In addition, Guo et al., have studied the adsorption of 2-mercaptobenzothiazole (MBT) on pyrite by SHINERS [93]. Au@SiO 2 nanoparticles were employed and deposited on pyrite for SHINERS experiments. From the SHINERS spectra recorded with an MBT concentration of 0.01 mM, a Raman peak at 1406 cm −1 was observed and corresponded to NCS ring stretch mode (see Figure 3a). This Raman peak suggested that double "minerophilic" groups of MBT were bound to pyrite surfaces in the configuration displayed in Figure 3b on the left. From the SHINERS spectra recorded with an MBT concentration of 0.1 mM, two Raman peaks at 1389 and 1409 cm −1 were observed and also corresponded to NCS ring stretch mode (see Figure 3c). The Raman peak at 1409 cm −1 has indicated that double "minerophilic" groups of MBT were bound to pyrite surface. The Raman peak at 1389 cm −1 (starting to appear at −200 mV, see Figure 3c) has also indicated that the MBT molecule was bound to pyrite with the exocyclic sulfur atom without the presence of any nitrogen-metal bond in the configuration displayed in Figure 3b on the right. In summary, the authors have concluded that the configuration of MBT molecules was preferentially the one in Figure 3b on the left for weaker concentrations of MBT and negative potentials and the one in Figure 3b on the right for higher concentrations of MBT and positive potentials [93].  (111) and polycrystalline Cu (Cu(poly)) surfaces by using the SHINERS technique [94]. Au@SiO 2 nanoparticles were employed for studying the oxidation mechanism of Cu(111) and Cu(poly) surfaces (see Figure 4a). From the SHINERS spectra for Cu (111)   The next three works have addressed the topic of oxygen reduction reaction (ORR) on Pt(hkl) surfaces by employing SHINERS spectroscopy. At first, Galloway et al., have demonstrated the surface specificity of the ORR on Pt(hkl) surfaces by SHINERS in sodiumoxygen electrochemistry [95]. The reduction of NaO 2 to Na 2 O 2 was favored on Pt (111) and Pt(110) surfaces in 0.1 M NaClO 4 dissolved in dimethyl sulfoxide (DMSO), whereas this reduction was not detected on Pt(100) and Pt(poly) surfaces (no characteristic Raman peak of Na 2 O 2 recorded) due to the restricted interactions with adsorbed oxygens [95]. Next, the second work realized by Dong (111) surface was obtained by the generation of OOH species, while for Pt (110) and Pt(100) surfaces by the formation of OH species [96]. Finally, the same group has demonstrated the presence of intermediate species during ORR on high-index Pt(hkl) surfaces by SHINERS spectroscopy [97]. Au@SiO 2 nanoparticles were used for the study of the ORR activity of these Pt surfaces (see Figure 5a). The authors have observed intermediate species for the two Pt surfaces (Pt(311) and Pt(211)) studied for ORR. From SHINERS spectra recorded for different values of potential, two characteristic Raman peaks at ∼765 and ∼1041 cm −1 were observed and corresponded to OOH and OH species, respectively (see Figure 5b,c). Moreover, they concluded that the Pt(211) surface had a better reactivity than the Pt(311) surface due to the greater adsorption energy for OOH species with the Pt(311) surface [97]. Barlow et al. have investigated the corrosion of 304 stainless steel by using the SHINERS technique. Au@SnO 2 nanoparticles were employed for examining this possible corrosion [98]. For the 304 stainless steel, the authors have evidenced a characteristic Raman (SHINERS) peak corresponding to amorphous Fe(OH) 2 , and also another Raman peak attributed to Cr(VI)-O bindings from a blended oxide based on Cr(VI). When KCl is present in the electrolyte, a Raman peak attributed to γ-FeOOH was observed. Finally, the authors have evidenced no green rust, i.e., no intermediate species during the conversion from Fe(OH) 2 to γ-FeOOH [98]. Besides, Wang et al., have reported on the effects of the size and the nature of nanocatalysts on the hydrogenation of para-nitrothiophenol (pNTP) by employing the SHINERS spectroscopy [99]. Au@SiO 2 nanoparticles were used for SHINERS experiments on which Pt, PtCu, PtNi nanocatalysts have been assembled via electrostatic interactions. At first, the authors have studied the size effect of Pt nanocatalysts on the pNTP hydrogenation, and have reported on an optimal size of 6.8 nm for Pt nanoparticles. Then, the authors have studied the kinetics of reaction for Pt, PtCu and PtNi nanocatalysts. They observed that PtCu and PtNi nanocatalysts have shown a quicker and quasi-complete conversion of pNTP than for Pt ones [99]. To finish this section on SHINERS applications to other surface catalytic reactions, Qiu et al. have investigated the effect of the presence of g-C 3 N 4 nanodots (CNNDs) and electrochemical exfoliated graphene layers (EGLs) on the photodegradation of Rhodamine B (RhB) molecules probed by SHINERS technique [100]. For SHINERS experiments, Ag@SiO 2 nanoparticles and an illumination wavelength of 632.8 nm were employed for examining this photocatalytic process of degradation of RhB molecules (see Figure 6a). In the absence of CNNDs and EGLs on the ITO substrate, the authors have noted no significant degradation of the intensity of the Raman peaks of RhB molecules. In contrast, the authors have observed a complete degradation of the intensity of these Raman peaks for an illumination time of 20 min with the presence of CNNDs and EGLs (see Figure 6b) [100].

SHINERS Application for the Detection of Molecules and Ions
In this final section for the SHINERS applications, we report on a couple of works on the detection of molecules and ions (see Table 3) [101][102][103][104][105][106][107].  [107] Wondergem et al. have reported the detection of Rhodamine 6G (R6G) molecules by SHINERS spectroscopy [101]. For SHINERS experiments, (Au/SiO 2 )@SiO 2 nanoparticles were used in order to avoid contact between gold nanoparticles and the liquid medium. The authors have obtained a detection limit of 10 −12 M for R6G molecules with these (Au/SiO 2 )@SiO 2 plasmonic superstructures. Moreover, these plasmonic superstructures can enable the study of catalytic reactions in liquids by using SHINERS [101]. In addition, Zhang et al. have investigated the photoinduced behavior of dyes (N719) molecules on three rutile TiO 2 (hkl) surfaces [102]. Au@SiO 2 nanoparticles were employed for SHINERS experiments. The authors have remarked that the SCN group of N719 molecules was the group that primarily adsorbed on these three rutile TiO 2 (hkl) surfaces. The authors have evidenced a shift of the Raman peak corresponding to the SCN group after an illumination time of 36 min on TiO 2 (110) and TiO 2 (001) surfaces, whereas no shift of this Raman peak was observed for the TiO 2 (111) surface. They concluded that the N719 molecules adsorbed on TiO 2 (111) surface were very stable in the time, whereas the N719 molecules had desorbed on TiO 2 (110) and TiO 2 (001) surfaces caused by the cleavage of the S = C binding [102]. Furthermore, Sun et al., have reported a detection limit of 10 −9 M for thiram molecules (pesticides) probed by the SHINERS spectroscopy [103]. Ag@SiO 2 nanoparticles on filter paper were employed as well as a miniaturized portable Raman analyzer based on smart-phone for SHINERS experiments. The authors have recorded SHINERS spectra for each concentration of thiram molecules (concentration range = 10 −9 -10 −3 M), where four characteristic Raman peaks of thiram molecules located at 440, 559, 1145 and 1379 cm −1 are displayed (see Figure 7). By using the Raman peak at 1379 cm −1 , the authors have deduced the detection limit (see Figure 7). In the next two works realized by the same group [104,105], the detection of copper ions and their oxidation states by SHINERS is addressed. Firstly, Forato et al., have investigated the detection of Cu(II) ions by SHINERS spectroscopy. Ag@TiO 2 nanoparticles and three excitation wavelengths (514, 633, and 785 nm) were used for SHINERS experiments. The authors have demonstrated an optimal efficiency for the detection of the Raman peak of the Cu-N binding at an excitation wavelength of 633 nm [104]. Then, Quéffelec et al., have reported on the distinctness of Cu(I) and Cu(II) ions by SHINERS measurements [105]. The authors have used the same Ag@TiO 2 nanoparticles functionalized with 2,2 -bipyridine phosphonate (bpy-PA) and an excitation wavelength of 633 nm where the efficiency was optimal [104]. From the SHINERS spectra recorded for Ag@TiO 2 @bpy-PA, Ag@TiO 2 @bpy-PA-Cu(I) and Ag@TiO 2 @bpy-PA-Cu(II), the authors have detected the characteristic Raman peaks of N-Cu(II) and N-Cu(I) vibrational modes at 230 and 290 cm −1 , respectively (see Figure 8). Krajczewski et al., have reported the detection of four-mercaptobenzoic acid (p-MBA) by SHINERS measurements [106]. Au@Ag concave cubic nanoparticles (noted: cc-Au@Ag NPs) were employed for the p-MBA detection probed by SHINERS measurements. The authors have demonstrated that these cc-Au@Ag NPs have improved by 35% the enhancement factor of the Raman signal of p-MBA molecules compared to the cc-Au NPs without the thin Ag shell. Moreover, the authors have added a thin layer of SiO 2 (<5 nm) on cc-Au@Ag NPs, and they recorded a reduction by 50% of the Raman signal of p-MBA molecules [106]. El-Said et al., have demonstrated the detection of neurotransmitters, such as γ-aminobutyric acid (GABA), by using the SHINERS technique [108]. Au@PPy nanobipyramids were used for this investigation, and allowed the detection of GABA with high sensitivity (detection limit of 116 nM). Moreover, these Au@PPy nanobipyramids have also enabled the detection of GABA in the presence of human serum, representing a real sample [108]. Zdaniauskienė et al., have used the SHINERS technique in order to study Metschnikowia pulcherrima yeast cells [109]. Au@SiO 2 nanoparticles were employed for this investigation, and allowed to obtain SHINERS spectra more enhanced than SERS spectra. Moreover, the Au@SiO 2 nanoparticles also allowed to suppress the appearance of supplementary bands due to potential interactions with the gold nanoparticles, and to identify the wall of yeast cells and their functional elements [109]. To conclude this section and also this review, two works related to cancer research, focusing on the identification of the atypical hyperplasia (AH) of the breast and the detection of tumor cells, are addressed. Zheng et al., have explored the identification of the breast AH by employing the SHINERS technique, which can provide a non-invasive diagnosis and study the cancer mechanisms at a molecular level [110]. The authors have remarked via changes in the Raman band intensities from SHINERS spectra that DNA strands have begun to snap in breast AH, and the presence of amino acid residues was more important than in normal breast tissues [110]. Nicinski et al., have reported on the improvement of the detection sensitivity of tumor cells such as renal cell carcinoma, and blood cells. This improvement was achieved by using Ag@SiO 2 nanoparticles via SHINERS measurements. The authors have observed variations in the intensities of Raman bands between cancer and healthy cells due to changes in the structure and quantity of molecules present during the formation of cancer cells [107].

Discussions and Future Directions
The shell-isolated nanoparticle-enhanced Raman spectroscopy has been generally employed with core-shell nanoparticles composed of gold cores (or silver) and silica shells, because the gold or silver cores presented a strong SERS activity. It would be interesting to use other well-known plasmonic materials, such as aluminum, copper, pal-ladium, and other alternative plasmonic materials (e.g., transparent conductive oxides and transition-metal nitrides) in order to study the influence of the nature of plasmonic material on the efficiency of the SHINERS technique to be analyzed different surface reactions at normal and high temperatures. Other materials for the shell fabrication, such as polymers, can be used to investigate the effect of shell material on this same efficiency cited previously. Through the three sections presented above, we have observed that the shell-isolated nanoparticle-enhanced Raman spectroscopy was non-invasive thanks to the catalytically inactive dielectric shell, and also had other advantages, such as the study of different surface catalytic reactions or adsorption of reactants on several surfaces of different natures and morphologies, and the detection of different molecules. Both these studies [84] were realized under normal conditions of temperature (T = 20-150 • C) and pressure. However, a great number of catalytic reactions are produced at high temperatures (T = 300-1000 • C) typically in industry. Thus, the thermal stability of SHINERS substrates should be improved for industrial applications. Moreover, the influence of high pressures on the stability of SHINERS substrates is still a research issue to be solved. Another challenging improvement to be addressed is to reduce the shell thickness (<1 nm; without pinhole in shell) in order to achieve a better Raman enhancement. Besides, another advantage would be to use the shell-isolated nanoparticle-enhanced Raman spectroscopy in catalysis as local nanosensors of molecules during the catalytic reactions in order to have a deep understanding of these catalytic reactions at the subnanometer scale [111]. Furthermore, the shell-isolated nanoparticle-enhanced Raman spectroscopy can be extended to other enhanced spectroscopies as tip-enhanced Raman spectroscopy (named shell-isolated TERS) [112] and sum-frequency generation spectroscopy (named SHINE-SFG) [113]. The shell-isolated TERS can allow the exclusion of interferences which are due to the presence of contaminants, and also the investigation of catalytic reactions at the level of a solid-liquid interface [112]. To finish, the SHINE-SFG spectroscopy can enable a novel type of enhancement coming from the non-linear coupling of SHINE-SFG with difference frequency generation. Thus, alternative substrates based on this type of coupling can be designed in order to enhance different signals [113].

Conclusions
In this mini-review, we addressed the applications of the shell-isolated nanoparticleenhanced Raman spectroscopy. We started with the SHINERS application to the reactions with CO molecules. The reactions of oxidation, hydrogenation, and adsorption of CO molecules with various catalysts have been presented. Next, we explored SHINERS studies on other surface catalytic reactions. Among these reactions, we presented a couple of works on oxygen reduction reactions realized on Pt(hkl) surfaces. Then, oxidation reactions of water and Cu surfaces have been exposed. Hydrogenation and photodegradation reactions, molecule adsorption, and corrosion have been also addressed. Finally, we reported on the detection of molecules and ions by SHINERS spectroscopy. The SHINERS experiments have enabled to improve the detection sensitivity of pesticides (thiram), tumor cells, and to distinguish the copper oxidation states. In conclusion, the shell-isolated nanoparticle-enhanced Raman spectroscopy can be very useful for obtaining various information on surface catalytic reactions, such as their mechanism and the intermediate species present during these reactions. Moreover, the SHINERS substrates based on coreshell nanoparticles can be employed as very sensitive nanosensors of molecules and ions.
Funding: This research received no external funding.

Conflicts of Interest:
The author declares no conflict of interest.