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This work studies the impact of the electrostatic interaction between analyte molecules and silver nanoparticles (Ag NPs) on the intensity of surface-enhanced Raman scattering (SERS). For this, we fabricated nanostructured plasmonic films by immobilization of Ag NPs on glass plates and functionalized them by a set of differently charged hydrophilic thiols (sodium 2-mercaptoethyl sulfonate, mercaptopropionic acid, 2-mercaptoethanol, 2-(dimethylamino)ethanethiol hydrochloride, and thiocholine) to vary the surface charge of the SERS substrate. We used two oppositely charged porphyrins, cationic copper(II) tetrakis(4-N-methylpyridyl) porphine (CuTMpyP4) and anionic copper(II) 5,10,15,20-tetrakis(4-sulfonatophenyl)porphine (CuTSPP4), with equal charge value and similar structure as model analytes to probe the SERS signal. Our results indicate that the SERS spectrum intensity strongly, up to complete signal disappearance, correlates with the surface charge of the substrate, which tends to be negative. Using the data obtained and our model SERS system, we analyzed the modification of the Ag surface by different reagents (lithium chloride, polyethylenimine, polyhexamethylene guanidine, and multicharged metal ions). Finally, all those surface modifications were tested using a negatively charged oligonucleotide labeled with Black Hole Quencher dye. Only the addition of copper ions into the analyte solution yielded a good SERS signal. Considering the strong interaction of copper ions with the oligonucleotide molecules, we suppose that inversion of the analyte charge played a key role in this case, instead of a change of charge of the substrate surface. Changing the charge of analytes could be a promising way to get clear SERS spectra of negatively charged molecules on Ag SERS-active supports.

This work aims at the impact of the electrostatic interaction between analyte molecules and silver nanoparticles (Ag NPs) on the surface enhanced Raman scattering (SERS) performance. For this, we fabricated nanostructured plasmonic films by immobilization of Ag NPs on glass plates and functionalized them by a set of differently charged hydrophilic thiols (sodium 2-mercaptoethyl sulfonate, mercaptopropionic acid, 2-mercaptoethanol, 2-(dimethylamino) ethanethiol hydrochloride and thiocholine) to vary the surface charge of the SERS-substrate. We used two oppositely charged porphyrins, cationic Cu(II)-tetrakis(4-N-methylpyridyl) porphine (CuTMpyP4) and anionic Cu(II)-5,10,15,20-tetrakis(4-sulphonatophenyl) porphine (CuTSPP4), with equal charge value and similar structure as model analytes to probe SERS signal. Our results indicate that the SERS spectrum intensity strongly, up to complete signal disappearance, correlates with the substrate’s surface charge that tends to be negative. Using the data obtained and our model SERS-system, we analyzed modification of Ag surface by different reagents (lithium chloride, polyethyleneimine, polyhexamethylene guanidine and multicharged metal ions). Finally, all those surface modifications were tested using a negatively charged oligonucleotide labeled with Black Hole Quencher (BHQ1) dye. Only addition of copper ions into the analyte solution allowed to get a good SERS signal. Considering strong interaction of copper ions with the DNA molecule, we suppose that the analyte charge inversion played the key role in that case, instead of the recharging of the substrate surface. Analyte recharging could be a promising way to get intensive SERS spectra of negatively charged molecules on Ag SERS-active supports.

The review reports on the results of our research work on nanostructured metal films onto porous silicon. Principal steps of the techniques allowing fabrication of metal films completely inheriting morphological pattern of different types of porous silicon are presented. It is shown, that giving of the nanostructured pattern to metal films by means of porous silicon template opens their new structural, optical, mechanical and electrical properties, which can be successfully applied in nanoelectronics and biomedicine, particularly including devices based on superconductivity effect, SERS analysis with picomolar sensitivity and transdermal drug delivery by electroporation.

The present work gives an overview of the developments in surface-enhanced Raman scattering (SERS) with metal-coated porous silicon used as an active substrate. We focused this review on the research referenced to SERS-active materials based on porous silicon, beginning from the patent application in 2002 and enclosing the studies of this year. Porous silicon and metal deposition technologies are discussed. Since the earliest studies, a number of fundamentally different plasmonic nanostructures including metallic dendrites, quasi-ordered arrays of metallic nanoparticles (NPs), and metallic nanovoids have been grown on porous silicon, defined by the morphology of this host material. SERS-active substrates based on porous silicon have been found to combine a high and well-reproducible signal level, storage stability, cost-effective technology and handy use. They make it possible to identify and study many compounds including biomolecules with a detection limit varying from milli- to femtomolar concentrations. The progress reviewed here demonstrates the great prospects for the extensive use of the metal-coated porous silicon for bioanalysis by SERS-spectroscopy.

A facile, one-step, and environmentally friendly fabrication of anisotropic gold nanostructures and size-controlled spherical silver nanoparticles (NP) using biopolymer pectin is reported. The reduction of Au and Ag ions was carried out at room temperature using an increasing concentration of pectin, which acts as the single source of reducing and stabilizing agent. The as-formed NPs were studied by UV-vis, infrared Fourier transform and surface-enhanced Raman spectroscopies, as well as transmission electron microscopy and energy dispersive X-ray spectroscopy. A high yield of anisotropic gold nanostructures was observed at low concentrations of pectin, while its increase results in the formation of smaller sharp edged perfect triangles with a considerable number of quasi-spherically shaped gold NP. On the other hand, the size of spherical silver NP decreased as the biopolymer concentration in the solution increased. The surface-enhanced Raman scattering enhancement of different NPs was evaluated using a Cu-complex of cationic tetrakis(4-N-methylpyridyl)porphyrin as a probe molecule at 441.6 and 532 nm excitation. Great enhancement of Raman signal was obtained with a pectin–silver NP and for most of them their levels were higher than that for the routinely synthesized citrate silver NP.

A highly sensitive method for the detection and quantitative evaluation of antimony(III) using the surface-enhanced Raman scattering (SERS) technique is demonstrated. The method is based on the analysis of SERS spectra intensity of antimony bound to phenylfluorone (Sb-PhF). Phenylfluorone is widely used as an organic reagent for the spectrophotometric determination of some heavy metals. For the SERS experiment a Sb-PhF complex was adsorbed onto the silvered porous silicon substrate. The significant degradation of the SERS signal was observed during measurements in the air. The time evolution of SERS spectra at ambient and degassed conditions was investigated to find an optimal regime for SERS measurements. The limit of Sb detection in degassed samples was determined to be near 1 ng/mL, which is one order of magnitude less than that attainable by the photometric approach. The linear range of the method to Sb(III) was found to a mass concentration range of 1–10 ng/mL. This approach permits an absolute quantity of Sb(III) to be detected at the picogram level (∼50 pg). It is remarkable that a very small sample volume (50 μL) is required for SERS analysis. Moreover this technique offers high selectivity owing to the distinctive vibrational features for the metallorganic complex and to the resonance character of Raman spectra. The proposed SERS-based detection of Sb is a fast and highly sensitive method for use in environmental and industrial waste monitoring as well as for forensic science to determine gunshot residue. We expect that the approach reported herein can be further extended to develop new detection techniques for other heavy metals.

Since its discovery surface enhanced Raman scattering (SERS) has been attracting a constantly increasing attention of the scientific community as this method demonstrates an extremely high sensitivity and allows detection of the lowest possible concentration of substances including biological specimens up to single molecule [1]. The enormous increase of the Raman signal is observed for the molecules adsorbed on the surfaces consisted of metallic nanoparticles (NPs), especially of the noble metals. The electromagnetic mechanism plays the most valuable role in such enhancement and is caused by plasmon resonance in the certain places on the metallic surfaces. Recently, use of ordered arrays of metallic nanovoids has been found to significantly improve reproducibility and enhancement of SERS signal due to specificity of the plasmon modes within metallic cavities which geometry is well controlled at the fabrication process [2, 3]. However, engineering of such arrays is often connected with an alternation of a number of liquid and dry operations which is desirable to avoid for technology simplification [4]. An urgent problem as well is still a development of the technology that provides an integration of SERS-active substrate with other elements on the single Si substrate. In the present work we propose fabrication of SERS-active metallic/macro porous silicon (PS) nanovoid arrays consisted of only electrochemical and electroless liquid steps. As an initial template we used PS formed by anodizing of p-Si (100) wafer in solution of HF:DMSO=10:46 at 8 mA/cm for 7 min. The depth and diameter of pores varied in the ranges 700-1200 nm and 500-1500 nm, respectively. It is in good agreement with the dimensions of nanovoids usually applied for SERS [24]. Two types of metallized nanovoids (single and hybrid) were fabricated by liquid deposition of Ag and Ni on PS. To form first type of substrate based on single metal (Ag), PS was immersed into the 3mM AgNO3+0.5M HF water solution for 40-200 min. As reaction of Ag reduction has positive standard potential, Ag ions attract electrons from Si atoms. In that way in water-based solutions Si is oxidized to SiO2 while Ag is reduced. The addition of HF provides removing of SiO2 for continuous supplying of Ag ions with electrons. To form hybrid metallization (Ag/Ni), Ni was electrochemically deposited on PS from the solution of 213 g/l NiSO4—7H2O+5 g/l NiCl2—6H2O+25 g/l H3BO3+3 g/l saccharin at 10 mA/cm 2 for 2.5 min. Such regime resulted in covering PS surface with continuous Ni film of 70 nm thickness. After that Ag was deposited by electroless method from the previously described solution for 5-30 min. The morphology and structure of the samples were studied with SEM Hitachi S-4800. SERS activity of substrates was tested using water-soluble CuTMpyP4 as an analyte compound. Raman spectra were registered with the spectrometer Solar TII DM160MS3504I, equipped with CCDdetector. The source of continuous excitation was the HeCd laser Liconix (λ=441.6 nm). Figure 1 shows typical spectra of CuTMpyP4 on the initial PS (a), Ag/PS (b) and Ag/Ni/PS (c). Both of metallized substrates demonstrated SERS-activity. However, hybrid metallization resulted in six orders of magnitude increase of the signal intensity in comparison to pure PS while Ag/PS provided enhancement of five orders of magnitude.

Porous silicon (PS) represents the prospective template for the fabrication of metal/PS nanostructures. However n+-type Sb doped Si Czochralski wafers widely used to fabricate PS may contain resistivity striations and as-grown microdefects with specific spiral distribution which influence Si dissolution during the formation of PS by anodizing. This may strongly affect on the PS microstructure and repeatability of metal/PS properties causing an increase of risk of a device failure. In the present research we studied the morphology and SERS-activity of Ag/PS nanostructures formed by Ag immersion deposition on PS. SEM investigation revealed unequal distribution of pore diameters in the surface regions of PS which corresponds to spiral swirl-like distribution of resistivity revealed in Si wafer by anodizing. Morphological changes of Ag deposit on spiral areas were found to cause decreasing of intensity of SERS signal. Additional implantation of Sb ions followed by high temperature annealing has shown to be a technique to improve homogeneity of Ag/PS SERS substrates. (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Plasmonic nanostructures demonstrating an activity in the surface-enhanced Raman scattering (SERS) spectroscopy have been fabricated by an immersion deposition of silver nanoparticles from silver salt solution on mesoporous silicon (meso-PS). The SERS signal intensity has been found to follow the periodical repacking of the silver nanoparticles, which grow according to the Volmer-Weber mechanism. The ratio of silver salt concentration and immersion time substantially manages the SERS intensity. It has been established that optimal conditions of nanostructured silver layers formation for a maximal Raman enhancement can be chosen taking into account a special parameter called effective time: a product of the silver salt concentration on the immersion deposition time. The detection limit for porphyrin molecules CuTMPyP4 adsorbed on the silvered PS has been evaluated as 10(-11) M.

It is found that the triplet state lifetimes of the series of nonplanar saddle-shaped porphyrins are much shorter than those of their planar analogs (hundreds of nanoseconds vs. several milliseconds). As our calculations show, two low-lying conformations with strongly different triplet-ground singlet state energy gap DETS exist for each molecule in its triplet state. An energy inversion of these conformations for the most distorted porphyrin molecule is found to be responsible for the observed non-monotonic dependence of the triplet state deactivation rate on the degree of nonplanar distortion.

SERS-active arrays of bimetallic nanovoids have been fabricated by Ni electrochemical plating followed by Ag immersion deposition on macroporous silicon (PS). Presence of HF in the solution for Ag deposition has been found to result in fabrication of the SERS-active substrates with 20 times enhanced Raman intensity in comparison with those formed in HF-free solution. The detection limit of R6G adsorbed on the bimetallic substrates has been determined to reach 10 M.

5,15-Bis(4-N-methylpyridyl)-10,20-diphenylporphyrin (5,15-DiMPyP, also known as t-H2Pagg) is capable of forming long-range ordered DNA-templated aggregates in solution. The present work reports the study of fluorescent properties of 5,15-DiMPyP, both monomeric and aggregated in presence of DNA, performed by means of steady-state and time-resolved fluorescence spectroscopy. The S1 state of free monomeric 5,15-DiMPyP is affected by a low-lying intramolecular charge transfer state which is destabilized upon porphyrin monomer binding to DNA. The aggregates, DNA-templated and random, of 5,15-DiMPyP were found to be emissive in solution and had respective S1 state lifetimes of 1.8 ns and 5.2 ns.

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The application of porous silicon as a template for the fabrication of nanosized copper objects is reported. Three different types of nanostructures were formed by displacement deposition of copper on porous silicon from hydrofluoric acid-based solutions of copper sulphate: (1) copper nanoparticles, (2) quasi-continuous copper films, and (3) free porous copper membranes. Managing the parameters of porous silicon (pore sizes, porosity), deposition time, and wettability of the copper sulphate solution has allowed to achieve such variety of the copper structures. Elemental and structural analyses of the obtained structures are presented. Young modulus measurements of the porous copper membrane have been carried out and its modest activity in surface enhanced Raman spectroscopy is declared.

Photophysical parameters of a set of metallocomplexes of meso-phenylthiazinylporphyrins with Zn (II), Pd (II) and Cu (II) ions were studied in different organic solvents, solid solutions and polymeric matrices at room and liquid nitrogen temperatures. The dependence of the spectral and photophysical parameters on changing the molecular structure with increasing number of branched substituents attached to aryl groups in different positions of the porphyrin macrocycle has been established.