Surface-enhanced resonance Raman spectroscopy of porphyrin and metalloporphyrin species in systems with Ag nanoparticles and their assemblies

Surface amplification of tetraphenylporphyrin overtone and combination Raman bands in drop coating deposition Raman (DCDR) on electrically conductive surfaces

It is essential to realize a Raman measurement technique without artifact or fluorescence signals for high-quality and reliable data in a valid molecular-level analysis and interpretation. This requirement applies especially to a molecule with strong fluorescence like porphyrin. Here, the surface of a gold substrate performs better as a DCDR substrate for tetraphenylporphyrin than other surfaces, such as tantalum, indium tin oxide glass, or aluminium. Polarized Raman spectra of tetraphenylporphyrin demonstrated the oriented deposition of porphyrin crystallites on the Au substrate using the drop coating technique. The emission anisotropy suggests that the deposited crystallites are arranged outward radially with the porphyrin ring orientation. The orientation is signed by the NHHN axis that is parallel to the radial vector along the X-axis. Moreover, it also demonstrates high chemical stability after preservation and repeated measurements. The Raman signal on a gold substrate is enhanced more than on other substrates beyond mere preconcentration of analytes or the coffee-ring effect only, which might be due to the contribution of the SERRS effect. This effect will be discussed based on the interactions among localized surface plasmons, vibronic transitions, and Raman active vibrational modes.

Distinctly Different Morphologies of Bimetallic Au-Ag Nanostructures and Their Application in Submicromolar SERS-Detection of Free Base Porphyrin

Core-shell Au-Ag nanostructures (Au-AgNSs) are prepared by a seed-meditated growth, i.e., by a two-step process. The synthetic parameters greatly influence the morphologies of the final bimetallic Au-AgNSs, their stability and application potential as surface-enhanced Raman scattering (SERS) substrates. Direct comparison of several types of Au NPs possessing different surface species and serving as seeds in Au-AgNSs synthesis is the main objective of this paper. Borohydride-reduced (with varying stages of borohydride hydrolysis) and citrate-reduced Au NPs were prepared and used as seeds in Au-AgNSs generation. The order of reactants in seed-mediated growth procedure represents another key factor influencing the final Au-AgNSs characteristics. Electronic absorption spectra, dynamic light scattering, zeta potential measurements, energy dispersive spectroscopy and transmission electron microscopy were employed for Au-AgNSs characterization. Subsequently, possibilities and limitations of SERS-detection of unperturbed cationic porphyrin, 5,10,15,20-tetrakis(1-methyl-4-pyridyl)21H,23H-porphine (TMPyP), were investigated by using these Au-AgNSs. Only the free base (unperturbed) SERS spectral form of TMPyP is detected in all types of Au-AgNSs. It reports about a well-developed envelope of organic molecules around each Au-AgNSs which prevents metalation from occuring. TMPyP, attached via ionic interaction, was successfully detected in 10 nM concentration due to Au-AgNSs.

Meso-tetra(4-sulfonatophenyl)porphyrin silver/Ag nanoparticles/graphene-phase C3N4 with a sandwich-like structure and double-faced active centers via two-step room-temperature photocatalytic synthesis for ractopamine detection

Photochemical synthesis under visible light irradiation is a novel approach in the field of green chemistry, and composites with abundant active centers for electrochemical detection are highly attractive. Herein, a meso-tetra(4-sulfonatophenyl)porphyrin silver/Ag nanoparticles/graphene phase C₃N₄ nanosheets (Ag₂TPPS₄/AgNPs/ng-C₃N₄) material with a sandwich-like structure was synthesized using a two-step photocatalytic reaction at room temperature (25 °C). In the first visible light irradiation step and in the presence of a hole capture agent, Ag⁺ ions were photocatalytically reduced onto the surface of ng-C₃N₄ that was used as a photocatalyst. Then, the protons (H⁺) in the core of H₂TPPS₄ were substituted in situ by photo-oxidized Ag⁺ during the second visible light irradiation step and in the presence of an electron capture agent. The electrochemical response of Ag₂TPPS₄ and ng-C₃N₄ to ractopamine (RAC) results in the unique double-faced active centers of Ag₂TPPS₄/AgNPs/ng-C₃N₄, and the cores (AgNPs) are beneficial as bridges for the connection between Ag₂TPPS₄ and ng-C₃N₄ and for high-efficiency electron transfer. Hence, as-synthesized Ag₂TPPS₄/AgNPs/ng-C₃N₄ exhibits high sensitivity (a low detection limit of 5.1 × 10^-8 M, S/N = 3.0), a wide linear range (1 × 10^-7 to 1.2 × 10^-5 M), and long-term stability. Based on the experimental verification of the electrochemical dynamics and electrostatic attraction at the interface between the dual-active-center surface and RAC, the electrochemical mechanism has been clarified. Specifically, in the multi-cycle oxidation of RAC, the blue shift of specific UV-vis peaks also confirms the electrocatalytic oxidation of the two terminal hydroxyl groups of RAC. In brief, Ag₂TPPS₄/AgNPs/ng-C₃N₄ with a sandwich-like structure and double-faced active centers enhances the detection sensitivity and electrocatalytic efficiency towards RAC.

Photo-induced surface-enhanced Raman spectroscopy from a diphenylalanine peptide nanotube-metal nanoparticle template

UV irradiation of aligned diphenylalanine peptide nanotubes (FF-PNTs) decorated with plasmonic silver nanoparticles (Ag NPs) enables photo-induced surface-enhanced Raman spectroscopy. UV-induced charge transfer facilitates a chemical enhancement that provides up to a 10-fold increase in surface-enhanced Raman intensity and allows the detection of a wide range of small molecules and low Raman cross-section molecules at concentrations as low as 10-13 M. The aligned FF-PNT/Ag NP template further prevents photodegradation of the molecules under investigation. Our results demonstrate that FF-PNTs can be used as an alternative material to semiconductors such as titanium dioxide for photo-induced surface-enhanced Raman spectroscopy applications.

Effect of Ethanethiolate Spacer on Morphology and Optical Responses of Ag Nanoparticle Array-Single Layer Graphene Hybrid Systems

Single layer graphene (SLG) and two-dimensional (2-D) plasmonic Ag nanoparticle arrays assembled by chemisorption of ethanethiol (ET) molecules (AgNPs-ET) were employed as components of two types of hybrid systems designed for surface-enhanced Raman scattering (SERS) spectral probing of SLG localized in the vicinity of plasmonic NPs. Both hybrids were characterized by optical microscopy, transmission electron microscopy (TEM), surface plasmon extinction (SPE), and SERS microRaman spectral measurements at four excitation wavelengths spanning the 445-780 nm range. SERS spectral probing of the glass/SLG/AgNPs-ET hybrid prepared by overdeposition of SLG on glass by the array of ET-modified Ag NPs has shown that the chemisorbed ET acts as an efficient molecular spacer between SLG and Ag NPs surface which, in turn, enabled to obtain SERS spectra of SLG unperturbed by doping or strain. TEM imaging and SERS spectral probing of the second hybrid prepared by overdeposition of AgNPs-ET array on glass by SLG revealed removal of the adsorbed ET molecules and annealing of Ag NPs during the SLG deposition. The characteristics of the resulting glass/AgNPs/SLG hybrid system, namely (i) broad distribution of the annealed Ag NPs sizes and shapes, (ii) SPE curve covering the overall visible spectral region, (iii) absence of the ET spectral bands in SERS spectra, and (iv) fairly uniform SERS enhancement of the G and 2D mode of SLG in the 532-780 nm range in the straight sample geometry indicate that this hybrid can provide a suitable platform for investigation of the excitation wavelength dependence of combined SERS/GERS (graphene-enhanced Raman scattering) enhancement experienced by various molecular species brought into contact with SLG in this hybrid. Finally, weak optical effects attributed to increased reflectivity of SLG in the near field of Ag NPs arrays have been observed in the excitation wavelength dependence of the SERS spectra of both types of hybrid systems.

Probing strong optical fields in compact aggregates of silver nanoparticles by SERRS of protoporphyrin IX

TEM images and measurements of SERRS (surface-enhanced resonance Raman scattering) spectra as a function of the porphyrin concentrations in systems with unmodified and chloride-modified Ag nanoparticles and protoporphyrin IX (PPIX) are reported. TEM images have shown formation of compact aggregates in systems with chloride modified Ag nanoparticles, as opposed to systems with the unmodified particles constituted by isolated particles. SERRS spectra of PPIX as a function of PPIX concentration were measured and subjected to factor analysis. Two spectral components were identified and tentatively attributed to unperturbed PPIX and to Ag+ -PPIX surface species. Concentration value of the SERRS spectral detection limit of the latter species was determined to be nearly three orders of magnitude lower in the system with the compact aggregates than in the system with separated nanoparticles and achieves the value of 1 x 10(-10) M in a macrosampling Raman experiment. TEM images and SERRS-micro-Raman spectra of single compact aggregates of chloride-modified Ag nanoparticles incorporating PPIX molecules were acquired from a sample prepared by attachment of the aggregates to amine groups of derivatized, SiOx/formvar coated copper grids for TEM. The SERRS signal has shown large temporal fluctuations as well as variations from one aggregate to another. Within the signal fluctuations, a SERRS spectrum showing the characteristic bands of both SERRS spectral forms of PPIX and originating most probably from a few PPIX molecules located in hot spots in the interstices between the Ag nanoparticles, was obtained.

Citrate-reduced silver hydrosol modified with omega-mercaptoalkanoic acids self-assembled monolayers as a substrate for surface-enhanced resonance Raman scattering. A study with cytochrome c

A new citrate-reduced silver hydrosol coated with omega-mercaptoalkanoic acids (mercaptopropionic and mercaptoundecanoic acids) self-assembled monolayers was prepared and characterized with surface-enhanced Raman spectroscopy. The structure and the quality of the coating monolayers are discussed and compared to similar coated and uncoated silver hydrosols previously developed. As an application, the new hydrosol was used as a biocompatible and efficient metal substrate for a surface-enhanced resonance Raman scattering (SERRS) study of cytochrome c. The high-quality SERRS spectra reported of cytochrome c (obtained using only 1 microL of a micromolar cytochrome solution) are discussed and compared with data available from literature studies.

Semi-quantitative analysis of indigo carmine, using silver colloids, by surface enhanced resonance Raman spectroscopy (SERRS)

The application of surface enhanced resonance Raman spectroscopy (SERRS) to the semi-quantitative analysis of the dye, indigo carmine, has been examined using citrate-reduced silver colloids. Good linear correlations are observed for the dye band at 1580 cm(-1) in the concentration range 10(-7)-10(-5) and 10(-9)-10(-5) mol dm(-3), using laser exciting wavelengths of 514.5 [(R=0.9983)] and 632.8 nm [(R=0.9978)], respectively. At concentrations of dye above 10(-6) M the concentration dependence of the SERRS signals is non-linear due to the coverage of the surface of the colloidal particles by the dye being in excess of a full monolayer. At concentrations above 10(-6) M resonance Raman spectroscopy (RRS) can be employed for the quantitative analysis of the dye. An internal standard was used and a good linear correlation (R=0.997) was observed for the dependence of dye signal intensities at 1580 cm(-1) in the concentration range 10(-5)-10(-4) M using a laser exciting wavelength of 514.5 nm. The limits of detection of indigo carmine by SERRS (514.5 nm), SERRS (632.8 nm) and solution RRS (514.5 nm) are found to be 0.9, 1 and 38 ppm, respectively.

Semi-quantitative analysis of indigo by surface enhanced resonance Raman spectroscopy (SERRS) using silver colloids

In this paper we report for the first time semi-quantitative analysis of indigo using surface enhanced Raman spectroscopy (SERS) and surface enhance resonance Raman spectroscopy (SERRS). Indigo, a dye widely used today in the textile industry, has been used, historically, both as a dye and as a pigment; the latter in both paintings and in printed material. The molecule is uncharged and largely insoluble in most solvents. The application of SERS/SERRS to the semi-quantitative analysis of indigo has been examined using aggregated citrate-reduced silver colloids with appropriate modifications to experimental protocols to both obtain and maximise SERRS signal intensities. Good linear correlations are observed for the dependence of the intensities of the SERRS band at 1151 cm(-1) using laser exciting wavelengths of 514.5 nm (R=0.9985) and 632.8 nm (R=0.9963) on the indigo concentration over the range 10(-7)-10(-5) and 10(-8)-10(-5) mol dm(-3), respectively. Band intensities were normalised against an internal standard (silver sol band at 243 cm(-1)). Resonance Raman spectra (RRS) of aqueous solutions of indigo could not be collected because of its low solubility and the presence of strong fluorescence. It was, however, possible to obtain RS and RRS spectra of the solid at each laser excitation wavelength. The limits of detection (L.O.D.) of indigo by SERS and SERRS using 514.5 and 632.8 nm were 9 ppm at both exciting wavelengths. Signal enhancement by SERS and SERRS was highly pH dependent due to the formation of singly protonated and possibly doubly protonated forms of the molecule at acidic pH. The SERS and SERRS data provide evidence to suggest that an excess of monolayer coverage of the dye at the surface of silver colloids is observed at concentrations greater than 7.85x10(-6) mol dm(-3) for each exciting wavelength. The data reported herein also strongly suggest the presence of multiple species of the indigo molecule.