Petr Šmejkal

Testing anionic spacers by SERRS (surface-enhanced resonance Raman scattering) of a cationic free-base porphyrin in systems with laser-ablated Ag colloids

Abstract Thiopheneacetic acid and several thiol-based anionic species were tested as prospective spacers between the surface of Ag colloidal particles (prepared by laser ablation) and a chromophoric cationic adsorbate. Time-evolution of the SERRS signal of a cationic free base porphyrin, 5,10,15,20-tetrakis(1-methyl-4-pyridiniumyl)porphine (H 2 TMPyP) attached to the surface of laser-ablated Ag colloidal particles modified by adsorption of either 3-thiopheneacetic acid or a thiol-based anionic species was measured on 2 h time-scale and analyzed. While H 2 TMPyP adsorbed on bare surfaces of laser-ablated Ag colloidal particles is rapidly converted into AgTMPyP surface complex (spectrally identical with its synthetic analogue), denaturation of the native structure of the porphyrin by formation of the same AgTMPyP surface species was not observed upon adsorption on the modified colloidal surfaces. The native structure of H 2 TMPyP was fully preserved upon adsorption on Ag colloid modified by 3-thiopheneacetic acid and good quality SERRS spectra of H 2 TMPyP were obtained from Ag colloid/3-thiopheneacetic acid/H 2 TMPyP system. By contrast, none of the thiol-based spacers enabled to obtain SERRS spectrum of unperturbed H 2 TMPyP. Modification of Ag colloid by 3-mercaptopropionic acid, 2-mercaptoethanesulfonic acid, 2-mercaptoethanesulfonate and 3-mercaptopropanesulfonate did not prevent metalation of the porphyrin, however, it affected the structure of the resulting AgTMPyP surface complex, as witnessed by the difference of the characteristic marker bands (384, 1005, 1366 and 1566 cm −1 ) from those of synthetically prepared AgTMPyP. Modification of the colloid by mercaptoethanol prevents porphyrin metalation, however, it induces another kind of perturbation of the native structure of H 2 TMPyP.

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

The advantages of systems with Ag nanoparticles and their assemblies for surface-enhanced resonance Raman scattering (SERRS) spectral investigation, detection and determination of porphyrin species are demonstrated. SERRS spectral detection limits of the testing porphyrin species (including porphyrin aggregates) in these systems are shown to be, on average, 10(2)-10(3) lower than detection limits by resonance Raman scattering (RRS). Systems with Ag nanoparticles modified by anionic organosulfur spacers enable us to obtain SERRS spectra of unperturbed cationic porphyrin species. In the case of thiopheneacetate-modified Ag particles prepared by laser ablation, no negative effect of the spacer on the spectral detection limit of the porphyrin was observed. Systems with isolated Ag nanoparticles allow for obtaining SERRS spectra of porphyrin species upon excitation into the Soret electronic absorption band which leads to at least a 10-fold decrease in the detection limit.

Characterization and surface-enhanced Raman spectral probing of silver hydrosols prepared by two-wavelength laser ablation and fragmentation

A four step Ag foil laser ablation-Ag nanoparticle fragmentation procedure in ultrapure water was carried out both under argon and in air. Pulses of a high power Nd/YAG laser were used for laser ablation (1064 nm) and for the three step Ag hydrosol treatment in the absence of Ag foil in the sequence 1064-532-1064 nm. Transmission electron microscopy (TEM) and surface plasmon (SP) extinction spectra provide evidence of Ag nanoparticle fragmentation in the second and third step of the procedure carried out under argon. While polydispersity of Ag hydrosol increases in the second step, both the polydispersity and the mean size of the nanoparticles are reduced in the third step. Qualitative and quantitative surface-enhanced Raman scattering (SERS)/surface-enhanced resonance Raman scattering (SERRS) spectral probing of systems with Ag hydrosols and the selected adsorbates at 514.5 nm excitation shows that Ag hydrosols obtained in the second step of the preparation procedure carried out in air are the most suitable substrates for SERS/SERRS experiments performed at this excitation wavelength.

Laser ablation of silver in aqueous ambient: effect of laser pulse wavelength and energy on efficiency of the process

Laser ablation (LA) of a Ag target in ultrapure water has been performed with nanosecond laser pulses of 355, 532 and 1064 nm in the range of fluences achievable for the particular wavelength. Efficiency of LA process was quantified in terms of the amount of ablated Ag as determined by atomic absorption spectroscopy (AAS) as well as of the area of the surface plasmon extinction (SPE) band of the resulting Ag nanoparticle hydrosol, and a fairly good agreement between the results produced by the two methods was obtained. Sigmoidally shaped plots of the LA efficiency as a function of laser fluence were obtained for LA with all wavelengths of laser pulses examined. Nevertheless, the maximum amount of Ag transferred from the target into the aqueous medium (yielding Ag nanoparticle hydrosol) is substantially (at least 6.5 x) larger for LA performed with 1064 nm pulses than that with 532 nm and 355 nm pulses. On the other hand, polydispersity of the hydrosol ablated with 1064 nm pulses is higher than that of the sols obtained with 532 and 355 nm pulses, most probably due to a limited extent of Ag nanoparticle fragmentation.

Preparation of Ag nanoparticles by two-wavelength laser ablation and fragmentation

The work is focused on the refinement of the laser ablation method for a nanosized silver colloid preparation with regards to the nanoparticle size distribution, as well as to the reproducibility and stability of the ablated hydrosol. In the adopted procedure, additionally to the variation of the laser pulse energy, beam focusation and duration of laser ablation, we developed a technique based on further fragmentation of the ablated colloid solution by subsequent treatment with the 1064 and 532 nm nanosecond laser pulses. The method yields redistribution of nanoparticle diameters to a smaller mean size, as it was observed by transmission electron microscopy, quasielastic light scattering studies and surface plasmon (SP) optical extinction. It was shown that a similar fragmentation procedure is also effective in reduction of nanoparticle size and polydispersity of silver hydrosol prepared by routine chemical procedures and leads to a narrower SP extinction profile.

Nanocomposites With Strong Optical Resonances: Silver Nanoparticles-Organic Molecules Systems

The ability of selected molecular species to link Ag nanoparticles into dimers and/or small aggregates has been tested. Dimercaptocarborane and ethidium bromide have been shown to link Ag nanoparticles via their bonding to Ag nanoparticle surface probably by the two strongly argentophilic groups in para-positions. Alternatively, dimers and small aggregates were assembled through an electrostatic interaction between negatively charged citrate-modified and positively charged polylysine-modified Ag nanoparticles, and a subsequent incorporation of 5, 10, 15, 20-tetrakis(4-sulphonato-phenyl)porphine (TSPP) into such preprepared nanoobjects has been probed by SERRS (surface-enhanced resonance Raman scattering). Formation of dimers and small aggregates has been established by TEM (transmission electron microscopy) and SEM (scanning electron microscopy). SE(R)RS spectral measurements from specific locations of samples containing molecularly-linked dimers and aggregates have shown temporal fluctuations (blinking) of the SE(R)RS signal, which indicates, that the signal likely originates from molecules located in the strong, nanoscale localized optical fields dubbed hot spots. In addition to that, characteristic bands of graphitic carbon were observed in the spectra and their intensities (together with the spectral background intensities) strongly varied with time and from one spectrum to another. One of the possible explanations of these observations is a photochemical and/or thermal decomposition of the molecules located in hot spots combined with diffusion of unperturbed molecules into hot spots.Copyright

Laser-ablated Ag colloids as efficient carriers of 3-thiopheneacetic acid spacer for SERRS of unperturbed free-base cationic porphyrin

The absence of residual anions on surfaces of Ag colloidal particles prepared by laser ablation was proved to be of key importance for reproducibility of metalation of a free-base cationic porphyrin, TMPyP [1]. The same testing SERRS experiment indicates that surfaces of the laser-ablated colloids are highly reactive towards Ag-TMPyP surface complex formation. We probe the ability of 3-thiopheneacetic acid (TAA) to modify the surface of laser-ablated Ag colloidal particles and prevent metalation of TMPyP [2]. We than apply the same test to a conventionally prepared, borohydride-reduced Ag colloid. In this contribution, the results obtained are quantitatively evaluated.