Influence of bulky substituents on single-molecule SERS sensitivity.

The surface-enhanced Raman spectroscopy (SERS) detection limit strongly depends on the molecular structure, which we demonstrate for a family of tert-butyl-substituted porphycenes. Even though the investigated species present very similar photophysical properties, the ratio between the SERS signal and fluorescence background depends on the number of bulky tert-butyl groups. Moreover, the probability of single molecule detection systematically drops with the number of the moieties attached to the pyrrole ring. As steric hindrance is the only significantly changing feature among the studied chromophores, we attribute the observed phenomena to the spatial structure. We also show that the sensitivity of the SERS technique can be improved by lowering the temperature. We managed to observe single-molecule spectra for derivatives for which this was unattainable at room temperature.

Unusual effects in single molecule tautomerization: hemiporphycene.

Parent hemiporphycene, a recently obtained constitutional isomer of porphyrin, exists in room temperature solutions and polymer matrices in the form of two trans tautomers interconverting via double hydrogen transfer. Using confocal fluorescence microscopy, it was possible to monitor tautomerization in single hemiporphycene molecules embedded in a PMMA film by monitoring the spectral and temporal evolution of their fluorescence spectra. The emission spectra of the two tautomeric forms are similar to those obtained from ensemble studies. However, the analysis of temporal spectral evolution reveals effects not detected in the bulk. For some single molecules, a large decrease of tautomerization rate was observed. This is interpreted as an indication of multidimensional character of the tautomerization coordinate and coupling of the reaction with the polymer relaxation processes. In addition, fluorescence lifetimes obtained for single molecules are significantly shorter than those measured for the bulk. It is proposed that the shortening is caused by environment-induced distortion of the molecule, which enhances the S0← S1 internal conversion rate by lowering the barrier to excited state single hydrogen transfer. This effect seems to reflect the specific morphology of thin (30 nm) polymer samples, because it is not observed in ensemble studies carried out using thick (tens of micrometers or more) PMMA films.

Substituent and solvent effects on the excited state deactivation channels in anils and boranils.

Differently substituted anils (Schiff bases) and their boranil counterparts lacking the proton-transfer functionality have been studied using stationary and femtosecond time-resolved absorption, fluorescence, and IR techniques, combined with quantum mechanical modelling. Dual fluorescence observed in anils was attributed to excited state intramolecular proton transfer. The rate of this process varies upon changing solvent polarity. In the nitro-substituted anil, proton translocation is accompanied by intramolecular electron transfer coupled with twisting of the nitrophenyl group. The same type of structure is responsible for the emission of the corresponding boranil. A general model was proposed to explain different photophysical responses to different substitution patterns in anils and boranils. It is based on the analysis of changes in the lengths of CN and CC bonds linking the phenyl moieties. The model allows predicting the contributions of different channels that involve torsional dynamics to excited state depopulation.

Tautomerization in 2,7,12,17-tetraphenylporphycene and 9-amino-2,7,12,17-tetraphenylporphycene: influence of asymmetry on the direction of the transition moment.

Femtosecond transient absorption anisotropy studies have been performed for two porphycenes of different symmetry. In 2,7,12,17-tetraphenylporphycene, the chemical identity of two trans forms implies a change in the S(0)-S(1) transition-moment direction upon tautomerization. Exploiting this phenomenon, the rates of double hydrogen transfer in both the S(0) and S(1) states (1.4×10(12)  s(-1) and 2.7×10(11)  s(-1) , respectively) have been determined by performing time-resolved anisotropy studies. In the asymmetric 9-amino-2,7,12,17-tetraphenylporphycene, tautomerization occurs with a similar rate in the ground state. In the S(1) state, the reaction is hindered in its vibrationally relaxed form, but the excitation spectra suggest that it may occur for an unrelaxed molecule. Unlike all porphycenes that have been studied so far, 9-amino-2,7,12,17-tetraphenylporphycene does not reveal significant changes in anisotropy owing to intramolecular double hydrogen transfer; rather, the transition-moment directions are similar in the two tautomeric forms. Analysis of the molecular orbitals allows for an explanation of the "locking" of the transition moments: it is due to a large splitting of the two HOMO orbitals, which retain the order of their energies in the two tautomers.

Near-Field Spectral Response of Optically Excited Scanning Tunneling Microscope Junctions Probed by Single-Molecule Action Spectroscopy.

The near-field spectral response of metallic nanocavities is a key characteristic in plasmon-assisted photophysical and photochemical processes. Here, we show that the near-field spectral response of an optically excited plasmonic scanning tunneling microscope (STM) junction can be probed by single-molecule reactions that serve as a nanoscale sensor detecting the local field intensity. Near-field action spectroscopy for the cis ↔ cis tautomerization of porphycene on a Cu(110) surface reveals that the field enhancement in the STM junction largely depends on microscopic structures not only on the tip apex, but also on its shaft. Using nanofabrication of Au tips with focused ion beam, we show that the spectral response is strongly modulated through the interference between the localized surface plasmon in the junction and propagating surface plasmon polariton generated on the shaft. Furthermore, it is demonstrated that the near-field spectral response can be manipulated by precisely shaping the tip shaft.

Plasmon-Mediated Surface Engineering of Silver Nanowires for Surface-Enhanced Raman Scattering.

We reveal nanoscale morphological changes on the surface of a silver nanowire (AgNW) in the conventional surface-enhanced Raman scattering (SERS) measurement condition. The surface morphology changes are due to the surface plasmon-mediated photochemical etching of silver in the presence of certain Raman probes, resulting in a dramatic increase of Raman scattering intensity. This observation indicates that the measured SERS enhancement does not always originate from the as-designed/fabricated structures themselves, but sometimes with contribution from the morphological changes by plasmon-mediated photochemical reactions. Our work provides a guideline for more reliable SERS measurements and demonstrates a novel method for simple and site-specific engineering of SERS substrate and AgNW probes for designing and fabricating new SERS systems, stable and efficient TERS mapping, and single-cell SERS endoscopy.

Single molecule Raman spectra of porphycene isotopologues.

Single molecule surface-enhanced resonance Raman scattering (SERRS) spectra have been obtained for the parent porphycene (Pc-d0) and its deuterated isotopologue (Pc-d12), located on gold and silver nanoparticles. Equal populations of "hot spots" by the two isotopologues are observed for 1 : 1 mixtures in a higher concentration range of the single molecule regime (5 × 10(-9) M). For decreasing concentrations, hot spots are preferentially populated by undeuterated molecules. This is interpreted as an indication of a lower surface diffusion coefficient of Pc-d12. The photostability of single Pc molecules placed on nanoparticles is strongly increased in comparison with polymer environments. Trans tautomeric species dominate the spectra, but the analysis of time traces reveals transient intermediates, possibly due to rare cis tautomeric forms.

An electropolymerized molecularly imprinted polymer for selective carnosine sensing with impedimetric capacity.

A chemosensor with a molecularly imprinted polymer (MIP) film as the recognition unit selective to a carnosine biomarker was molecularly engineered, devised and fabricated. The molecular structure of the pre-polymerization complex of the carnosine template with the carboxy and 18-crown-6 ether derivatives of bis(2,2'-bithien-5-yl)methane functional monomers was thermodynamically optimized by density functional theory (DFT) at the B3LYP/6-31g(d) level. The calculated high negative Gibbs free energy change, ΔG = -227.4 kJ mol-1, indicated the formation of a very stable complex. The solution of this complex was prepared and used for deposition of the MIP films on a Pt disk electrode or an Au electrode of the quartz crystal resonator by potentiodynamic electropolymerization. Subsequently, the carnosine template was extracted from the MIPs with 0.1 M NaOH, as confirmed by the differential pulse voltammetry (DPV), X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy measurements. For carnosine sensing, impedimetric capacity (IC) measurements were performed under flow-injection analysis (FIA) conditions resulting in the limit of detection of 20 µM (at S/N = 3). This limit implied the readiness of the chemosensor for carnosine determination in clinical samples. Due to multiple modes of carnosine binding to MIP recognition sites, the IC chemosensor was found to be more selective to carnosine than to its common interferences including anserine, carcinine and histidine. Advantageously, the imprinting factor, determined by piezoelectric microgravimetry (PM), was high equaling 14.9.