Jingting Li

High-speed hyperspectral Raman imaging for label-free compositional microanalysis

We present high-speed hyperspectral Raman imaging with integrated active-illumination for label-free compositional microanalysis. We show that high-quality Raman spectra can be acquired from as many as ~1,000 spots/sec semi-randomly distributed among a ~100x100 μm(2) area without mechanical scanning. We demonstrate rapid data acquisition from three types of samples: 1) uniform, strong Raman scatterers, e.g., silicon substrates; 2) non-uniform, medium-strength Raman scatterers, e.g., polymer microparticles; and, 3) non-uniform, relatively weak Raman scatterers, e.g., bacterial spores. We compare the system performance to that of point-scan with an electron-multiplied CCD camera, as implemented in some commercial systems. The results suggest that our system not only provides significant imaging speed advantage for various types of samples, but also permits substantially longer integration time per spot, leading to superior signal-to-noise ratio data. Our system enables the rapid collection of high quality Raman spectra for reliable and robust compositional microanalysis that are potentially transformative in applications such as semiconductor material and device, polymer blend and biomedicine.

Raman spectroscopy as a diagnostic tool for monitoring acute nephritis.

Both acute nephritis and chronic nephritis account for substantial morbidity and mortality worldwide, partly due to the lack of reliable tools for detecting disease early and monitoring its progression non-invasively. In this work, Raman spectroscopy coupled with multivariate analysis are employed for the first time to study the accelerated progression of nephritis in anti-GBM mouse model. Preliminary results show up to 98% discriminant accuracy for the severe and midly diseased and the healthy among two strains of mice with different susceptibility to acute glomerulonephritis. This technique has the potential for non-invasive or minimally-invasive early diagnosis, prognosis, and monitoring of renal disease progression.

Direct-write patterning of nanoporous gold microstructures by in situ laser-assisted dealloying

We report a novel patterning technique to direct-write microscale nanoporous gold (NPG) features by projecting laser patterns using a spatial light modulator (SLM) onto an Au/Ag alloy film immersed in diluted nitric acid solutions. Heat accumulation induced by the photothermal effect enables localized dealloying in such solutions, which is otherwise impotent at room temperature. Consequently, NPG micropatterns are formed at the irradiated spots while the surrounding alloy remains intact. We have studied the size of the patterned NPG microstructures with respect to laser power and irradiation time. The NPG microstructures become significantly more transparent compared to the original alloy film. The NPG microstructures also exhibit strong localized surface plasmon resonance (LSPR) which is otherwise weak in the original alloy film. Both the light transmission intensity and LSPR peak wavelength have been demonstrated to be sensitive to the local environmental refractive index as quantified by microscopy and spectroscopy.

Catalytic assembly of DNA nanostructures on a nanoporous gold array as 3D architectures for label-free telomerase activity sensing

Telomerase, an enzyme known to catalyze telomere elongation by adding TTAGGG [thymine (T), adenine (A), and guanine (G)] repeats to the end of telomeres, is vital for cell proliferation. Overexpression of telomerase has been found in most tumor cells, resulting in telomere dysfunction and uncontrolled cellular proliferation. Thus, telomerase has been considered as a potential cancer biomarker, as well as a potential target in cancer therapy. In this study, telomerase-catalyzed growth of tandem G-quadruplex (G4) assembled on a nanoporous gold array (NPGA) resulted in the formation of three-dimensional hybrid nanoarchitectures. The generated nanostructure then captured malachite green (MG) (reporter molecule) without the need of a complicated labeling process. Upon laser irradiation, the captured MG molecules produced a surface-enhanced Raman scattering (SERS) signal that was generated by an abundant amount of plasmonic hot spots in the NPGA substrates. A limit of detection (LOD) of 10-10 IU along with a linear range, which was 3 orders of magnitude, was achieved, which was equivalent to the telomerase amount extracted from 20 HeLa cells. The LOD is 2 orders of magnitude better than that of the commercial enzyme-linked immunosorbent assay (ELISA), and it approaches that of the most sensitive technique, telomeric repeat amplification protocols (TRAP), which require a laborious and equipment-intensive polymerase chain reaction (PCR). In addition, X-ray photoelectron spectroscopy (XPS) was used to chemically identify and quantify the telomerase activity on the sensitized NPGA surface. Furthermore, the sensor was applied to screen the effectiveness of anti-telomerase drugs such as zidovudine, thus demonstrating the potential use of the sensor in telomerase-based diagnosis and drug development. Moreover, the framework represents a novel paradigm of collaborative plasmonic intensification and catalytic multiplication (c-PI/CM) for label-free biosensing.

Photothermal generation of programmable microbubble array on nanoporous gold disks

We present a technique to generate microbubbles using photothermal effects induced by continuous-wave laser irradiation of random nanoporous gold disk (NPGD) array covered microfluidic channels. The conversion efficiency is higher than 50% and the photothermally generated microbubbles can be used for microfluidic controls and assembly.

A flexible and rapid frequency selective scheme for SRS microscopy

Stimulated Raman scattering (SRS) is a label-free imaging technique suitable for studying biological systems. Due to stimulated nature by ultrafast laser pulses, SRS microscopy has the advantage of significantly higher sensitivity but often reduced spectroscopic information. In this paper, we present a newly constructed femtosecond SRS microscope with a high-speed dynamic micromirror device based pulse shaper to achieve flexible and rapid frequency selection within the C-H stretch region near 2800 to 3100 cm-1 with spectral width of 30 cm-1. This technique is applicable to lipid profiling such as cell activity mapping, lipid distribution mapping and distinction among subclasses.

Portable SERS sensor for malachite green and other small dye molecules

Sensitive detection of specific chemicals on site can be extremely powerful in many fields. Owing to its molecular fingerprinting capability, surface-enhanced Raman scattering has been one of the technological contenders. In this paper, we describe the novel use of DNA topological nanostructure on nanoporous gold nanoparticle (NPG-NP) array chip for chemical sensing. NPG-NP features large surface area and high-density plasmonic field enhancement known as “hotspots”. Hence, NPG-NP array chip has found many applications in nanoplasmonic sensor development. This technique can provide novel label-free molecular sensing capability and enables high sensitivity and specificity detection using a portable Raman spectrometer.

Laser-assisted dealloying for direct-write patterning of plasmonic nanostructures

Recently, nanoporous gold (NPG) has attracted significant interest due to its unique properties such as large specific surface area, bi-continuous nanostructure, high electrical conductivity and the applicability of thiol-gold surface chemistry. Patterned NPG disks showcase tunable pore and ligament sizes ranging from nanometers to microns. The nanoporous structure and sub-wavelength nanoparticle shape contribute to its unique LSPR properties. NPG disk not only features large specific surface area, but high-density plasmonic field enhancement known as “hot-spots”. Hence, NPG disks have found many applications in nanoplasmonic sensor development. In our recent studies, we have shown that NPG disks array chip can be utilized for high-sensitivity detection by various enhanced spectroscopic modalities, as photothermal agents, and for disease biomarker detection. To date, patterned NPG disks have been exclusively fabricated by colloidal nanosphere lithography. Starting with pattern transfer into alloy disks, dealloying subsequently turns the alloy disks into NPG disks. In this paper, we present another NPG patterning method by localized laser heating, during which dealloying occurs at the laser focal spots due to elevated temperature. This approach has enabled us to pattern NPG entity with various sizes and shapes. We have investigated fabrication parameters such as laser power, irradiation duration, and solution environment. We have also characterized the plasmonic resonance of the patterned NPG disks by extinction spectroscopy. The noncontact nature of this technique is well suited for the processing of substrates immersed in an aqueous environment. Further, this technique shares the same advantages as maskless laser direct writing.

Sensitive and selective nanoplasmonic sensor by functionalized nanoporous gold nanoparticle array chip

Nanoplasmonic sensor has become a recent research focus due to its significant signal enhancement and robust signal transduction measured by various techniques. However, since the native gold surface does not have the capability to selectively bind target biomolecules, high molecular specificity has been a challenge. Nanoporous gold nanoparticle (NPG-NP) array chip showcases large specific surface area and high-density plasmonic field enhancement known as “hot-spots”. In this paper, we discuss strategies to enhance molecular specificity by functionalizing NPG-NP with unique bio-recognition elements towards both high sensitivity and specificity. A few examples will be given using existing and novel bio-recognition elements.

Monitoring adsorption of gold nanoparticles on gold nanodisk array using dark-field hyperspectral microscopy (Conference Presentation)

Localized surface plasmon resonance (LSPR) arises from the interaction of light with noble metal nanoparticles, which induces a collective oscillation in the free electrons. The size and shape of the metallic nanostructure significantly impact LSPR frequency and strength. Nanoplasmonic sensor has become a recent research focus due to its significant signal enhancement and robust signal transduction measured by extinction spectroscopy, fluorescence, Raman scattering, and absorption spectroscopy. Dark-field microscopy, in contrast, reports the scattered photons after light-matter interactions. In this case, the nanoparticles can be understood as dipole radiators whose free electrons oscillate in concert. Coupled with spectroscopy, this platform allows the collection of plasmonically scattered spectra from gold nanoparticles. Plasmonic coupling between electron-beam lithography patterned gold nanodisks (AuND) and colloidal gold nanoparticles (AuNP) can change the plasmonic resonance of the original entities, and can be effectively studied by dark-field hyperspectral microscopy. Typically, a pronounced redshift can be observed when plasmonic coupling occurs. When these nano-entities are functionalized with interactive surface moieties, biochemistry and molecular processes can be studied. In this paper, we will present the capability of assessing the process of immobilizing streptavidin-functionalized AuNPs on an array of biotin-terminated AuNDs. By monitoring changes in the LSPR band of AuNDs, we are able to evaluate similar processes in other molecular systems. In addition, plasmon coupling induced scattering intensity variations can be measured by an electron-multiplied charge-coupled device camera for rapid in situ monitoring. This method can potentially be useful in studying dynamic biophysical and biochemical processes in situ.