Juen-Kai Wang

Functionalized arrays of Raman-enhancing nanoparticles for capture and culture-free analysis of bacteria in human blood

Summary form only given. Detecting bacteria in clinical samples without the time-consuming culture process is most desired for rapid diagnosis. Such a culture-free detection needs to capture and analyze bacteria from a body fluid usually containing complicate constituents. Here we show that vancomycin (Van) coating of a special substrate with arrays of Ag-nanoparticles, which can provide label-free analysis of bacteria via surface enhanced Raman spectroscopy (SERS), leads to 1000 folds increase in its capability to capture bacteria without introducing significant spectral interference. Bacteria spiked in human blood can be concentrated onto a microscopic Van-coated area while blood cells are excluded. Furthermore, A Van-coated substrate provides distinctly different SERS spectra of Van-susceptible and Van-resistant Enterococcus, indicating its potential use for drug-resistance test. Our results represent a critical step towards the creation of SERS-based multifunctional biochips for rapid culture/label-free detection and drug-resistant testing of microorganisms in clinical samples.

A High Speed Detection Platform Based on Surface-Enhanced Raman Scattering for Monitoring Antibiotic-Induced Chemical Changes in Bacteria Cell Wall

Rapid and accurate diagnosis for pathogens and their antibiotic susceptibility is critical for controlling bacterial infections. Conventional methods for determining bacteriums sensitivity to antibiotic depend mostly on measuring the change of microbial proliferation in response to the drug. Such “biological assay” inevitably takes time, ranging from days for fast-growing bacteria to weeks for slow-growers. Here, a novel tool has been developed to detect the “chemical features” of bacterial cell wall that enables rapid identification of drug resistant bacteria within hours. The surface-enhanced Raman scattering (SERS) technique based on our newly developed SERS-active substrate was applied to assess the fine structures of the bacterial cell wall. The SERS profiles recorded by such a platform are sensitive and stable, that could readily reflect different bacterial cell walls found in Gram-positive, Gram-negative, or mycobacteria groups. Moreover, characteristic changes in SERS profile were noticed in the drug-sensitive bacteria at the early period (i.e., ∼1 hr) of antibiotic exposure, which could be used to differentiate them from the drug-resistant ones. The SERS-based diagnosis could be applied to a single bacterium. The high-speed SERS detection represents a novel approach for microbial diagnostics. The single-bacterium detection capability of SERS makes possible analyses directly on clinical specimen instead of pure cultured bacteria.

Charge carrier mobility of mixed-layer organic light-emitting diodes

The authors report the investigation of the charge transport behaviors in mixed thin films of N,N′-diphenyl-N,N′-bis(1-napthyl)-1,1′-biphenyl-4,4′-diamine and tris(8-hydroxyquinoline) aluminum. The extracted electron and hole drift mobility were found to be sensitive to the compositional fraction and interpreted by energy levels, charge mobilities of neat compounds, and microscopic networks within the mixed systems. The carrier conduction characteristics, therefore, were used to illustrate the electrical and optical properties of the organic light emitting devices with a mixed layer and present direct evidences on the role of the mixed layer in these devices.

Effects of cathode buffer layers on the efficiency of bulk-heterojunction solar cells

The effects of cathode buffer layers on the bulk-heterojunction solar cells are investigated. Comparing with the device without buffer layer, obvious enhancements of Voc from 0.38 to 0.65 V and fill factor from 44% to 63% have been achieved by using 2 nm bathocuproine layer, which make the efficiency of the devices improved from 1.63% to 4.11%. Alternatively, lithium fluoride and/or tris(8-hydroxyquinolinato) aluminum were also introduced for clarification purpose. X-ray photoelectron spectroscopy study indicates that the degradation caused by the outer diffusion of carbon from active layers plays a crucial role in the device performance.

Looking into meta-atoms of plasmonic nanowire metamaterial.

Nanowire-based plasmonic metamaterials exhibit many intriguing properties related to the hyperbolic dispersion, negative refraction, epsilon-near-zero behavior, strong Purcell effect, and nonlinearities. We have experimentally and numerically studied the electromagnetic modes of individual nanowires (meta-atoms) forming the metamaterial. High-resolution, scattering-type near-field optical microscopy has been used to visualize the intensity and phase of the modes. Numerical and analytical modeling of the mode structure is in agreement with the experimental observations and indicates the presence of the nonlocal response associated with cylindrical surface plasmons of nanowires.

Light scattering from 2D arrays of monodispersed Ag-nanoparticles separated by tunable nano-gaps: spectral evolution and analytical analysis of plasmonic coupling

Two dimensional arrays of monodispersed Ag-nanoparticles separated by different gaps with sub-10 nm precision are fabricated on anodic alumina substrates with self-organized pores. Light scattering spectra from the arrays evolve with the gaps, revealing plasmonic coupling among the nanoparticles, which can be satisfactorily interpreted by analytical formulae derived from generic dipolar approximation. The general formulism lays down a foundation for predicting the Q factor of an array of metallic nano-particles and its geometric characteristics.

Electric field enhancement by a nanometer-scaled conical metal tip in the context of scattering-type near-field optical microscopy

We present a numerical study of the electric field enhancement in the immediate vicinity of the apex of a conical silver tip and show that an optimal cone angle exists, allowing one to maximize the electric field. This angle depends on the tip length, the wavelength, as well as on the distance from the apex to the observation point. So both the angle and length of the tip can be considered as parameters to adjust the peak enhancement resonant position for a laser source wavelength. At the same time, reducing the cone angle does not ensure a concurrent increase in the electric field enhancement. A simple qualitative interpretation is proposed to explain this phenomenon based on competition of two mechanisms affecting the electric field near the tip apex. The results obtained show that the point-like dipole approximation is invalid for description of the field enhancement of a finite-size metal tip in the case of scattering-type near-field optical microscopy. One more conclusion is that the model of a sharp...

Transparent Raman-enhancing substrates for microbiological monitoring and in?situ pollutant detection

Opaque Raman-enhancing substrates made of Ag nanoparticles on incompletely oxidized aluminum templates have been rendered transparent by an ion-drift process to complete the oxidation. The result shows that the transparent substrates exhibit high/uniform surface-enhanced Raman scattering (SERS) capability and good optical transmissivity, allowing for concurrent SERS characterization and high contrast transmission-mode optical imaging of S. aureus bacteria. We also demonstrate that the transparent substrates can used in conjunction with optical fibers as SERS sensors for in situ detection of malachite green down to 10(-9) M.

Nonlinear refraction and absorption measurements with chirped femtosecond laser pulses: experiments and simulations

We report an extension of the spectrally resolved two-beam coupling technique to measure the nonlinear intensity index of refraction (n2I) and the two-photon absorption coefficient (β) by use of chirped laser pulses. The linear chirp parameter b is incorporated into the derivation of a more general model than the previous one [Opt. Lett.22, 1077 (1997)]. We have also analyzed the validity of this linear chirp model through a comparison of the experimental results for fused silica with the numerically accurate calculation that considers higher-order chirps obtained by second-harmonic generation frequency-resolved optical gating. The results show that this method potentially can be used to extract the chirp. Finally, we applied this transient spectrally resolved nonlinear transmittance spectroscopy to semiconductor-doped glasses to extract their n2I and β.

Rapid bacterial antibiotic susceptibility test based on simple surface-enhanced Raman spectroscopic biomarkers

Rapid bacterial antibiotic susceptibility test (AST) and minimum inhibitory concentration (MIC) measurement are important to help reduce the widespread misuse of antibiotics and alleviate the growing drug-resistance problem. We discovered that, when a susceptible strain of Staphylococcus aureus or Escherichia coli is exposed to an antibiotic, the intensity of specific biomarkers in its surface-enhanced Raman scattering (SERS) spectra drops evidently in two hours. The discovery has been exploited for rapid AST and MIC determination of methicillin-susceptible S. aureus and wild-type E. coli as well as clinical isolates. The results obtained by this SERS-AST method were consistent with that by the standard incubation-based method, indicating its high potential to supplement or replace existing time-consuming methods and help mitigate the challenge of drug resistance in clinical microbiology.