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1.
J Phys Chem A ; 2020 Mar 26.
Artigo em Inglês | MEDLINE | ID: mdl-32212703

RESUMO

Vibrational sum-frequency generation (SFG) spectroscopy is used to determine the surface pKa of p-methyl benzoic acid (pMBA) at the air-water interface, by monitoring the carbonyl and carboxylate stretching modes over the pH range from 2 to 12. The SFG intensities of pMBA and its conjugate base, p-methyl benzoate (pMBA-), exhibit an anomalously large enhancement over a narrow pH range (~0.5 units) centered at 6.3 near the SFG determined surface pKa, 5.9 ± 0.1. The increase in surface pKa relative to the bulk value of 4.34 is consistent with the trend previously observed for long chain carboxylic acids, in which the surface pKa is higher than the bulk solution pKa. SFG polarization studies help distinguish orientation and number density contributions to this observed anomalous surface phenomenon. The large SFG intensity increase is attributed to an increase in pMBA and pMBA- surface concentrations in this narrow pH range due to a cooperative adsorption effect between pMBA and pMBA-. This cooperativity is manifested only on the 2-D air-water interface, where the interactions between the acid and base are not as dielectrically screened as in the aqueous bulk phase. Surface effects are critical to understanding and controlling the reactivity, solubility and behavior of organic acids at interfaces, and can have an impact for biomedical applications.

2.
Nat Commun ; 10(1): 5318, 2019 11 21.
Artigo em Inglês | MEDLINE | ID: mdl-31754221

RESUMO

Stimulated Raman scattering (SRS) microscopy allows for high-speed label-free chemical imaging of biomedical systems. The imaging sensitivity of SRS microscopy is limited to ~10 mM for endogenous biomolecules. Electronic pre-resonant SRS allows detection of sub-micromolar chromophores. However, label-free SRS detection of single biomolecules having extremely small Raman cross-sections (~10-30 cm2 sr-1) remains unreachable. Here, we demonstrate plasmon-enhanced stimulated Raman scattering (PESRS) microscopy with single-molecule detection sensitivity. Incorporating pico-Joule laser excitation, background subtraction, and a denoising algorithm, we obtain robust single-pixel SRS spectra exhibiting single-molecule events, verified by using two isotopologues of adenine and further confirmed by digital blinking and bleaching in the temporal domain. To demonstrate the capability of PESRS for biological applications, we utilize PESRS to map adenine released from bacteria due to starvation stress. PESRS microscopy holds the promise for ultrasensitive detection and rapid mapping of molecular events in chemical and biomedical systems.

3.
Phys Chem Chem Phys ; 19(10): 6989-6995, 2017 Mar 08.
Artigo em Inglês | MEDLINE | ID: mdl-28244512

RESUMO

This paper reports the effects of substrate roughness on the odd-even effect in n-alkanethiolate self-assembled monolayers (SAMs) probed by vibrational sum frequency generation (SFG) spectroscopy. By fabricating SAMs on surfaces across the so-called odd-even limit, we demonstrate that differentiation of the vibrational frequencies of CH3 from SAMs derived from alkyl thiols with either odd (SAMO) or even (SAME) numbers of carbons depends on the roughness of the substrate on which they are formed. Odd-even oscillation in SFG susceptibility amplitudes was observed for spectra derived from SAME and SAMO fabricated on flat surfaces (RMS roughness = 0.4 nm) but not on rougher surfaces (RMS roughness = 2.38 nm). In addition, we discovered that local chemical environments for the terminal CH3 group have a chain-length dependence. There seems to be a transition at around C13, beyond which SAMs become "solid-like".

4.
Anal Chem ; 88(16): 8026-35, 2016 08 16.
Artigo em Inglês | MEDLINE | ID: mdl-27429301

RESUMO

Traditional methods for identifying pathogens in bacteremic patients are slow (24-48+ h). This can lead to physicians making treatment decisions based on an incomplete diagnosis and potentially increasing the patient's mortality risk. To decrease time to diagnosis, we have developed a novel technology that can recover viable bacteria directly from whole blood and identify them in less than 7 h. Our technology combines a sample preparation process with surface-enhanced Raman spectroscopy (SERS). The sample preparation process enriches viable microorganisms from 10 mL of whole blood into a 200 µL aliquot. After a short incubation period, SERS is used to identify the microorganisms. We further demonstrated that SERS can be used as a broad detection method, as it identified a model set of 17 clinical blood culture isolates and microbial reference strains with 100% identification agreement. By applying the integrated technology of sample preparation and SERS to spiked whole blood samples, we were able to correctly identify both Staphylococcus aureus and Escherichia coli 97% of the time with 97% specificity and 88% sensitivity.


Assuntos
Escherichia coli/isolamento & purificação , Staphylococcus aureus/isolamento & purificação , Humanos , Análise Espectral Raman/instrumentação , Propriedades de Superfície
5.
Anal Bioanal Chem ; 408(17): 4631-47, 2016 Jul.
Artigo em Inglês | MEDLINE | ID: mdl-27100230

RESUMO

The dominant molecular species contributing to the surface-enhanced Raman spectroscopy (SERS) spectra of bacteria excited at 785 nm are the metabolites of purine degradation: adenine, hypoxanthine, xanthine, guanine, uric acid, and adenosine monophosphate. These molecules result from the starvation response of the bacterial cells in pure water washes following enrichment from nutrient-rich environments. Vibrational shifts due to isotopic labeling, bacterial SERS spectral fitting, SERS and mass spectrometry analysis of bacterial supernatant, SERS spectra of defined bacterial mutants, and the enzymatic substrate dependence of SERS spectra are used to identify these molecular components. The absence or presence of different degradation/salvage enzymes in the known purine metabolism pathways of these organisms plays a central role in determining the bacterial specificity of these purine-base SERS signatures. These results provide the biochemical basis for the development of SERS as a rapid bacterial diagnostic and illustrate how SERS can be applied more generally for metabolic profiling as a probe of cellular activity. Graphical Abstract Bacterial typing by metabolites released under stress.


Assuntos
Bactérias/metabolismo , Metabolômica , Análise Espectral Raman/métodos , Marcação por Isótopo , Espectrometria de Massas por Ionização por Electrospray/métodos , Propriedades de Superfície
6.
ACS Photonics ; 1(8): 696-702, 2014 Aug 20.
Artigo em Inglês | MEDLINE | ID: mdl-25541620

RESUMO

We report on the mid-infrared nonlinear photothermal spectrum of the neat liquid crystal 4-octyl-4'-cyanobiphenyl (8CB) using a tunable Quantum Cascade Laser (QCL). The nonequilibrium steady state characterized by the nonlinear photothermal infrared response undergoes a supercritical bifurcation. The bifurcation, observed in heterodyne two-color pump-probe detection, leads to ultrasharp nonlinear infrared spectra similar to those reported in the visible region. A systematic study of the peak splitting as a function of absorbed infrared power shows the bifurcation has a critical exponent of 0.5. The observation of an apparently universal critical exponent in a nonequilibrium state is explained using an analytical model analogous of mean field theory. Apart from the intrinsic interest for nonequilibrium studies, nonlinear photothermal methods lead to a dramatic narrowing of spectral lines, giving rise to a potential new contrast mechanism for the rapidly emerging new field of mid-infrared microspectroscopy using QCLs.

7.
ACS Nano ; 5(12): 9898-906, 2011 Dec 27.
Artigo em Inglês | MEDLINE | ID: mdl-22077149

RESUMO

The exciton dynamics for an ensemble of individual, suspended (6,5), single-walled carbon nanotubes revealed by single color E(22) resonant pump-probe spectroscopy for a wide range of pump fluences are reported. The optically excited initial exciton population ranges from approximately 5 to 120 excitons per ∼725 nm nanotube. At the higher fluences of this range, the pump-probe signals are no longer linearly dependent on the pump intensity. A single, predictive model is described that fits all data for two decades of pump fluences and three decades of delay times. The model introduces population loss from the optically active zero momentum E(22) state to the rest of the E(22) subband, which is dark due to momentum selection rules. In the single exciton limit, the E(11) dynamics are well described by a stretched exponential, which is a direct consequence of diffusion quenching from an ensemble of nanotubes of different lengths. The observed change in population relaxation dynamics as a function of increasing pump intensity is attributed to exciton-exciton Auger de-excitation in the E(11) subband and, to a lesser extent, in the E(22) subband. From the fit to the model, an average defect density 1/ρ = 150 nm and diffusion constants D(11) = 4 cm(2)/s and D(22) = 0.2 cm(2)/s are determined.


Assuntos
Nanotubos de Carbono/química , Nanotubos de Carbono/ultraestrutura , Transferência de Energia , Dinâmica não Linear , Tamanho da Partícula , Água/química
8.
Appl Spectrosc ; 65(5): 493-9, 2011 May.
Artigo em Inglês | MEDLINE | ID: mdl-21513591

RESUMO

It has been recently suggested [N. E. Marotta and L. A. Bottomley, Appl. Spectrosc. 64, 601-606 (2010)] that previously reported surface-enhanced Raman scattering (SERS) spectra of vegetative bacterial cells are due to residual cell growth media that were not properly removed from samples of the lab-cultured microorganism suspensions. SERS spectra of several commonly used cell growth media are similar to those of bacterial cells, as shown here and reported elsewhere. However, a multivariate data analysis approach shows that SERS spectra of different bacterial species grown in the same growth media exhibit different characteristic vibrational spectra, SERS spectra of the same organism grown in different media display the same SERS spectrum, and SERS spectra of growth media do not cluster near the SERS spectra of washed bacteria. Furthermore, a bacterial SERS spectrum grown in a minimal medium, which uses inorganics for a nitrogen source and displays virtually no SERS features, exhibits a characteristic bacterial SERS spectrum. We use multivariate analysis to show how successive water washing and centrifugation cycles remove cell growth media and result in a robust bacterial SERS spectrum in contrast to the previous study attributing bacterial SERS signals to growth media.


Assuntos
Bactérias/química , Análise Espectral Raman/métodos , Bactérias/isolamento & purificação , Meios de Cultura/química , Análise de Componente Principal
9.
Lab Chip ; 10(23): 3265-70, 2010 Dec 07.
Artigo em Inglês | MEDLINE | ID: mdl-20938505

RESUMO

We report a low cost, disposable polymer microfluidic sample preparation device to perform rapid concentration of bacteria from liquid samples using enhanced evaporation targeted at downstream detection using surface enhanced Raman spectroscopy (SERS). The device is composed of a poly(dimethylsiloxane) (PDMS) liquid sample flow layer, a reusable metal airflow layer, and a porous PTFE (Teflon™) membrane sandwiched in between the liquid and air layers. The concentration capacity of the device was successfully demonstrated with fluorescently tagged Escherichia coli (E. coli). The recovery concentration was above 85% for all initial concentrations lower than 1 × 10(4) CFU mL(-1). In the lowest initial concentration cases, 100 µL initial volumes of bacteria solution at 100 CFU mL(-1) were concentrated into 500 nL droplets with greater than 90% efficiency in 15 min. Subsequent tests with SERS on clinically relevant Methicillin-Sensitive Staphylococcus aureus (MSSA) after concentration in this device proved more than 100-fold enhancement in SERS signal intensity compared to the signal obtained from the unconcentrated sample. The concentration device is straightforward to design and use, and as such could be used in conjunction with a number of detection technologies.


Assuntos
Bactérias/metabolismo , Dispositivos Lab-On-A-Chip , Dimetilpolisiloxanos/química , Eletroquímica/métodos , Desenho de Equipamento , Escherichia coli/metabolismo , Filtração , Corantes Fluorescentes/farmacologia , Meticilina/farmacologia , Modelos Estatísticos , Pressão , Análise Espectral Raman/métodos , Staphylococcus aureus/metabolismo , Células-Tronco , Propriedades de Superfície
10.
ACS Nano ; 3(5): 1190-202, 2009 May 26.
Artigo em Inglês | MEDLINE | ID: mdl-19354266

RESUMO

Defined nanoparticle cluster arrays (NCAs) with total lateral dimensions of up to 25.4 microm x 25.4 microm have been fabricated on top of a 10 nm thin gold film using template-guided self-assembly. This approach provides precise control of the structural parameters in the arrays, allowing a systematic variation of the average number of nanoparticles in the clusters (n) and the edge-to-edge separation (Lambda) between 1 < n < 20 and 50 nm < or = Lambda < or = 1000 nm, respectively. Investigations of the Rayleigh scattering spectra and surface-enhanced Raman scattering (SERS) signal intensities as a function of n and Lambda reveal direct near-field coupling between the particles within individual clusters, whose strength increases with the cluster size (n) until it saturates at around n = 4. Our analysis shows that strong near-field interactions between individual clusters significantly affect the SERS signal enhancement for edge-to-edge separations Lambda < 200 nm. The observed dependencies of the Raman signals on n and Lambda indicate that NCAs support a multiscale signal enhancement which originates from simultaneous inter- and intracluster coupling and |E|-field enhancement. The NCAs provide strong and reproducible SERS signals not only from small molecules but also from whole bacterial cells, which enabled a rapid spectral discrimination between three tested bacteria species: Escherichia coli, Bacillus cereus, and Staphylococcus aureus.


Assuntos
Cristalização/métodos , Ouro/química , Nanoestruturas/química , Nanoestruturas/ultraestrutura , Nanotecnologia/métodos , Análise Espectral Raman/métodos , Substâncias Macromoleculares/química , Conformação Molecular , Tamanho da Partícula , Propriedades de Superfície
11.
J Phys Chem B ; 112(40): 12776-82, 2008 Oct 09.
Artigo em Inglês | MEDLINE | ID: mdl-18793010

RESUMO

Ultrafast infrared spectroscopy of N 2O is shown to be a sensitive probe of hydrophobic and aqueous sites in lipid bilayers. Distinct rates of VER of the nu 3 antisymmetric stretching mode of N 2O can be distinguished for N 2O solvated in the acyl tail, interfacial water, and bulk water regions of hydrated dioleoylphosphatidylcholine (DOPC) bilayers. The lifetime of the interfacial N 2O population is hydration-dependent. This effect is attributed to changes in the density of intermolecular states resonant with the nu 3 band ( approximately 2230 cm (-1)) resulting from oriented interfacial water molecules near the lipid phosphate. Thus, the N 2O VER rate becomes a novel and experimentally convenient tool for reporting on the structure and dynamics of interfacial water in lipids and, potentially, in other biological systems.


Assuntos
Bicamadas Lipídicas/química , Sondas Moleculares/química , Óxido Nitroso/química , Vibração , Água/química , Simulação por Computador , Espectrofotometria
12.
J Phys Chem A ; 111(51): 13457-65, 2007 Dec 27.
Artigo em Inglês | MEDLINE | ID: mdl-18052050

RESUMO

A novel third-order polarization effect due to an accumulated optical heterodyne detected (OHD) transient acoustic grating response in near critical fluids was observed and experimentally characterized. Femtosecond pump-probe responses in near critical CO2 and CHF3 illustrate this phenomenon. This large optically generated acoustic response due to electrostrictive coupling appears only when pump and probe pulses are temporally overlapped and is pi out-of-phase with the normal optical Kerr effect (OKE) birefringent signal. The local oscillator, the laser intensity, and the modeled experimental repetition rate dependence identify the accumulated heterodyne origin of these responses. The observed OHD accumulated acoustic birefringent signal is inversely dependent on sound velocity to the fifth power. A corresponding sound velocity dependent dichroic (in-phase) response was also observed for these electronically nonresonant samples. The accumulated effect described here may have applications for the design of efficient modulators and as a simple and sensitive experimental technique for the measurement of near critical fluid thermodynamic and acoustic parameters.

13.
J Phys Chem B ; 111(40): 11824-31, 2007 Oct 11.
Artigo em Inglês | MEDLINE | ID: mdl-17880126

RESUMO

Recent studies demonstrate that photoactive proteins can react within several picoseconds to photon absorption by their chromophores. Faster subpicosecond protein responses have been suggested to occur in rhodopsin-like proteins where retinal photoisomerization may impulsively drive structural changes in nearby protein groups. Here, we test this possibility by investigating the earliest protein structural changes occurring in proteorhodopsin (PR) using ultrafast transient infrared (TIR) spectroscopy with approximately 200 fs time resolution combined with nonperturbing isotope labeling. PR is a recently discovered microbial rhodopsin similar to bacteriorhodopsin (BR) found in marine proteobacteria and functions as a proton pump. Vibrational bands in the retinal fingerprint (1175-1215 cm(-1)) and ethylenic stretching (1500-1570 cm(-1)) regions characteristic of all-trans to 13-cis chromophore isomerization and formation of a red-shifted photointermediate appear with a 500-700 fs time constant after photoexcitation. Bands characteristic of partial return to the ground state evolve with a 2.0-3.5 ps time constant. In addition, a negative band appears at 1548 cm(-1) with a time constant of 500-700 fs, which on the basis of total-15N and retinal C15D (retinal with a deuterium on carbon 15) isotope labeling is assigned to an amide II peptide backbone mode that shifts to near 1538 cm(-1) concomitantly with chromophore isomerization. Our results demonstrate that one or more peptide backbone groups in PR respond with a time constant of 500-700 fs, almost coincident with the light-driven retinylidene chromophore isomerization. The protein changes we observe on a subpicosecond time scale may be involved in storage of the absorbed photon energy subsequently utilized for proton transport.


Assuntos
Proteínas/análise , Proteínas/química , Rodopsina , Rodopsinas Microbianas , Espectrofotometria Infravermelho , Fatores de Tempo
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