nanomaterials Review Substrates for Surface-Enhanced Raman Scattering Formed on Nanostructured Non-Metallic Materials: Preparation and Characterization Jan Krajczewski, Robert Ambroziak and Andrzej Kudelski * Faculty of Chemistry, University of Warsaw, Pasteur Str. 1, 02-093 Warsaw, Poland; [email protected] (J.K.); [email protected] (R.A.) * Correspondence: [email protected]; Tel.: +48-2255-264-01 Abstract: The efficiency of the generation of Raman spectra by molecules adsorbed on some sub- strates (or placed at a very close distance to some substrates) may be many orders of magnitude larger than the efficiency of the generation of Raman spectra by molecules that are not adsorbed. This effect is called surface-enhanced Raman scattering (SERS). In the first SERS experiments, nanostructured plasmonic metals have been used as SERS-active materials. Later, other types of SERS-active materials have also been developed. In this review article, various SERS substrates formed on nanostructured non-metallic materials, including non-metallic nanostructured thin films or non-metallic nanopar- ticles covered by plasmonic metals and SERS-active nanomaterials that do not contain plasmonic metals, are described. Significant advances for many important applications of SERS spectroscopy of substrates based on nanostructured non-metallic materials allow us to predict a large increase in the significance of such nanomaterials in the near future. Some future perspectives on the application of SERS substrates utilizing nanostructured non-metallic materials are also presented. Keywords: SERS; surface-enhanced Raman spectroscopy; SERS substrates; multifunctional materials Citation: Krajczewski, J.; Ambroziak, R.; Kudelski, A. Substrates for Surface-Enhanced Raman Scattering 1. Introduction Formed on Nanostructured Non-Metallic Materials: Preparation In the 1970s, it was observed that the intensity of Raman spectra generated by and Characterization. Nanomaterials molecules adsorbed on some nanostructured surfaces is many orders of magnitude larger 2021, 11, 75. https://doi.org/ than the intensity of Raman spectra generated by the same number of molecules but not 10.3390/nano11010075 adsorbed. This effect was called surface-enhanced Raman scattering (SERS). In some cases, the generated SERS signal is so strong that it is possible to record a reliable SERS spectrum Received: 19 November 2020 even of a single molecule [1,2]. This means that the SERS spectroscopy is one of the most Accepted: 27 December 2020 sensitive analytical tools. Published: 31 December 2020 It is generally accepted that the enhancement of SERS spectra is due to the cooperation of two mechanisms: the electromagnetic and the chemical one. The electromagnetic Publisher’s Note: MDPI stays neu- mechanism is due to inducing the SERS substrate by the electromagnetic wave of the tral with regard to jurisdictional clai- electric dipole (see Figure1), which may lead to a significant local increase in the intensity ms in published maps and institutio- of the electric field. The magnitude of the induced dipole (p) is proportional to [3]: nal affiliations. e (n) − e (n) p ∼ M out (1) eM(n) + 2eout(n) Copyright: © 2020 by the authors. Li- where: n is the frequency of the excitation radiation, and eM(n) and eout(n) are the dielectric censee MDPI, Basel, Switzerland. functions of the metal and the surrounding medium, respectively. When the denominator This article is an open access article of the above fraction is close to zero, a strong electric dipole is induced, which leads distributed under the terms and con- to a very large local intensity of the electric field. This condition may be fulfilled in a ditions of the Creative Commons At- significant part of the spectrum of visible radiation, for example for silver, gold, and copper tribution (CC BY) license (https:// creativecommons.org/licenses/by/ nanoparticles. The SERS enhancement generated by this electromagnetic effect is roughly 4.0/). proportional to the fourth power of the field enhancement [4,5]. Nanomaterials 2021, 11, 75. https://doi.org/10.3390/nano11010075 https://www.mdpi.com/journal/nanomaterials Nanomaterials 2021, 11, x FOR PEER REVIEW 2 of 27 () −() ~ (1) () +2() where: ν is the frequency of the excitation radiation, and M(ν) and out(ν) are the dielectric functions of the metal and the surrounding medium, respectively. When the denominator of the above fraction is close to zero, a strong electric dipole is induced, which leads to a very large local intensity of the electric field. This condition may be fulfilled in a significant Nanomaterials 2021, 11, 75 part of the spectrum of visible radiation, for example for silver, gold, and copper nano- 2 of 25 particles. The SERS enhancement generated by this electromagnetic effect is roughly pro- portional to the fourth power of the field enhancement [4,5]. Figure 1. Schematic illustration of the surface plasmon resonance (SPR) phenomenon. Reproduced with permission [6]. Figure 1. Schematic illustration of the surface plasmon resonance (SPR) phenomenon. Reproduced with permission [6]. Copyright, 2020, Springer Nature. Copyright, 2020, Springer Nature. The second enhancement mechanism, which also leads to an increase in the efficiency of Thethe generation second enhancement of the Raman mechanism,signal, is called which the chemical also leads enhancement. to an increase The ininteraction the efficiency of theof adsorbed generation molecules of the with Raman the metal signal, substrate is called provides the chemical new electronic enhancement. transitions The for metal interaction of adsorbed(or adsorbed molecules molecule) withelectrons. the The metal electrons substrate at the provides Fermi level new of the electronic metal can transitions be virtu- for metalally (orexcited adsorbed into unoccupied molecule) molecular electrons. orbitals The electrons of the adsorbed at the Fermi molecule level and of theback metal to the can be virtuallymetal (see excited Figure into 2a), unoccupied or the electrons molecular at the highest orbitals occupied of the molecu adsorbedlar orbital molecule can be and vir- back to Nanomaterials 2021, 11, x FOR PEERthe REVIEWtually metal excited (see Figure into the2 a),Fermi or thelevel electrons of the metal at and the back highest to the occupied adsorbed molecular molecule (see orbital Figure can 3 be of 27 virtually2b). This excited means into that thea mechanism Fermi level analogous of the metal for the and standard back toresonance the adsorbed Raman molecule process (see Figure(that2 canb). Thisgenerate means a significant that a mechanism increase in the analogous efficiency for of thethe generation standard resonanceof the Raman Raman processsignal) (that may canbe observed generate for a adsorbed significant molecules increase [7]. in the efficiency of the generation of the do notTo contain record plasmonicthe SERS spectrum, metals. Some Raman future scatterers perspectives (studied ofmolecules) the application must be of ad- composite Raman signal) may be observed for adsorbed molecules [7]. SERSsorbed substrates at or be placed are also in a discussed. close proximity to the SERS-active material. The first surface- enhanced Raman spectra were published in 1974—in this case, the measurements were carried out on nanostructured silver [6]. During the next three decades, the SERS meas- urements were also carried out mainly on nanostructured plasmonic metals (Ag, Au and Cu), although other SERS substrates were also developed relatively quickly: in 1982, Loo recorded SERS spectra using as a SERS-active material polycrystalline TiO2 [8] and in 1983, Yamada and Yamamoto produced SERS-active materials by the deposition of some metals on the surface of NiO and TiO2 [9]. The number of SERS measurements carried out on composite substrates containing non-metallic nanostructured materials are still increas- ing. This is due to many significant advances of such SERS substrates in some applications of SERS spectroscopy, such composites usually have additional important functionalities that are not possible to obtain in standard SERS substrates produced only from plasmonic metals. In this review article, we present selected SERS substrates that utilize nanostruc- tured non-metallic materials, including non-metallic nanostructured thin films or non- metallic nanoparticles covered by plasmonic metals and SERS-active nanomaterials that FigureFigure 2. 2.Schematic Schematic illustration illustration of of the the energy energy levels levels involved involved in thein the charge charge transfer transfer (CT) (CT) mechanism mecha- nism of the surface-enhanced Raman scattering (SERS) enhancement; hν—photon energy, EF— of the surface-enhanced Raman scattering (SERS) enhancement; hν—photon energy, EF—Fermi Fermi level energy, ECT—energies of occupied and unoccupied levels of the adsorbed molecules. level energy, E —energies of occupied and unoccupied levels of the adsorbed molecules. (a) The (a) The electronsCT at the Fermi level of the metal are virtually excited into unoccupied molecular orbit- electronsals of the at adsorbed the Fermi molecule level of theand metal back to are the virtually metal, ( excitedb) the electrons into unoccupied at the highest molecular occupied orbitals molecu- of thelar adsorbed orbital can molecule be virtually and excited back to into the the metal, Fermi (b )level the electronsof the metal at theand highest back to occupiedthe adsorbed molecular mole- orbitalcule. canReproduced be virtually with excited permission