Jianbo Zeng

Microfluidic surface-enhanced Raman scattering sensor with monolithically integrated nanoporous gold disk arrays for rapid and label-free biomolecular detection

Abstract. We present a microfluidic surface-enhanced Raman scattering (SERS) sensor for rapid and label-free biomolecular detection. Our sensor design mitigates a common limiting factor in microfluidic SERS sensors that utilize integrated nanostructures: low-efficiency transport of biomolecules to nanostructured surface which adversely impacts sensitivity. Our strategy is to increase the total usable nanostructured surface area, which provides more adsorption sites for biomolecules. Specifically, a nanoporous gold disk (NPGD) array, a highly effective SERS substrate, has been monolithically integrated inside a microfluidic chip. Individual NPGD is known to feature an order of magnitude larger surface area than its projected disk area. The increased surface area arises from nanoscale pores and ligaments three-dimensionally distributed in the NPGD, which manifest themselves as high-density SERS hot-spots. High-density NPGD arrays further guarantee large coverage of these hot-spots on the microchannel floor. The sensor performance has been demonstrated using Rhodamine 6G to quantify spatial uniformity and determine the shortest detection time. Next, the sensor is applied to detect two biomolecules, dopamine and urea, with unprecedented detection limit and speed compared to other existing microfluidic SERS sensors. The sensor holds great promise in point-of-care applications for various biomolecular detections.

Internal and external morphology-dependent plasmonic resonance in monolithic nanoporous gold nanoparticles

We report morphology-dependent plasmonic resonance in monolithic nanoporous gold nanoparticles with a nanoscale internal porous network produced by the combination of lithographic patterning and dealloying. Timed dealloying and post-dealloying thermal annealing techniques have been employed to precisely control the morphological evolution. We found that prolonged dealloying time caused further pore coarsening to increase by ∼4 nm, whereas thermal annealing induced both pore coalescence and disk shrinkage, which eventually led to pore elimination. Both types of morphological changes caused a blueshift in the major plasmonic extinction band of up to 200 nm, in contrast to the redshift (∼50 nm) observed in semi-infinite NPG thin films. In addition, a greater blueshift was observed in a higher Au atomic content starting alloy. The tunable plasmonic properties have great potential in surface-enhanced spectroscopy and optical sensing.

Reagent- and separation-free measurements of urine creatinine concentration using stamping surface enhanced Raman scattering (S-SERS).

We report a novel reagent- and separation-free method for urine creatinine concentration measurement using stamping surface enhanced Raman scattering (S-SERS) technique with nanoporous gold disk (NPGD) plasmonic substrates, a label-free, multiplexed molecular sensing and imaging technique recently developed by us. The performance of this new technology is evaluated by the detection and quantification of creatinine spiked in three different liquids: creatinine in water, mixture of creatinine and urea in water, and creatinine in artificial urine within physiologically relevant concentration ranges. Moreover, the potential application of our method is demonstrated by creatinine concentration measurements in urine samples collected from a mouse model of nephritis. The limit of detection of creatinine was 13.2 nM (0.15 µg/dl) and 0.68 mg/dl in water and urine, respectively. Our method would provide an alternative tool for rapid, cost-effective, and reliable urine analysis for non-invasive diagnosis and monitoring of renal function.

Morphological control and plasmonic tuning of nanoporous gold disks by surface modifications

We report a surface modification protocol to control nanoporous gold (NPG) disk morphology and tune its plasmonic resonance. Enlarged pore size up to ∼20 nm within 60 s dealloying time has been achieved by adsorbing halides onto alloy surfaces in-between two dealloying steps. In addition, plasmonic resonance has been significantly red-shifted by up to ∼258 nm by the surface modification. Furthermore, with the enlarged pore size, small gold nanoparticles have been effectively loaded into the pores to enhance the performance of surface-enhanced Raman scattering (SERS) due to hot spot formation between the original nanoporous network and loaded nanoparticles.

Label-free, in situ SERS monitoring of individual DNA hybridization in microfluidics

We present label-free, in situ monitoring of individual DNA hybridization in microfluidics. By immobilizing molecular sentinel probes on nanoporous gold disks, we demonstrate sensitivity approaching the single-molecule limit via surface-enhanced Raman scattering which provides robust signals without photobleaching for more than an hour. We further demonstrate that a target concentration as low as 20 pM can be detected within 10 min under diffusion-limited transport.

Label‐free, zeptomole cancer biomarker detection by surface‐enhanced fluorescence on nanoporous gold disk plasmonic nanoparticles

We experimentally demonstrate a label-free biosensor for the ERBB2 cancer gene DNA target based on the distance-dependent detection of surface-enhanced fluorescence (SEF) on nanoporous gold disk (NPGD) plasmonic nanoparticles. We achieve detection of 2.4 zeptomole of DNA target on the NPGD substrate with an upper concentration detection limit of 1 nM. Without the use of molecular spacers, the NPGD substrate as an SEF platform was shown to provide higher net fluorescence for visible and NIR fluorophores compared to glass and non-porous gold substrates. The enhanced fluorescence signals in patterned nanoporous gold nanoparticles make NPGD a viable material for further reducing detection limits for biomolecular targets used in clinical assays. With patterned nanoporous gold disk (NPGD) plasmonic nanoparticles, a label-free biosensor that makes use of distance-dependent detection of surface-enhanced fluorescence (SEF) is constructed and tested for zeptomole detection of ERBB2 cancer gene DNA targets.

Nanoporous Gold Nanocomposites as a Versatile Platform for Plasmonic Engineering and Sensing

Plasmonic metal nanostructures have shown great potential in sensing applications. Among various materials and structures, monolithic nanoporous gold disks (NPGD) have several unique features such as three-dimensional (3D) porous network, large surface area, tunable plasmonic resonance, high-density hot-spots, and excellent architectural integrity and environmental stability. They exhibit a great potential in surface-enhanced spectroscopy, photothermal conversion, and plasmonic sensing. In this work, interactions between smaller colloidal gold nanoparticles (AuNP) and individual NPGDs are studied. Specifically, colloidal gold nanoparticles with different sizes are loaded onto NPGD substrates to form NPG hybrid nanocomposites with tunable plasmonic resonance peaks in the near-infrared spectral range. Newly formed plasmonic hot-spots due to the coupling between individual nanoparticles and NPG disk have been identified in the nanocomposites, which have been experimentally studied using extinction and surface-enhanced Raman scattering. Numerical modeling and simulations have been employed to further unravel various coupling scenarios between AuNP and NPGDs.

Label-free, multiplexed, molecular sensing and imaging by stamping SERS

Surface-enhanced Raman spectroscopy (SERS) is a spectroscopic technique, where Raman scattering is boosted primarily by enhanced electric field due to localized surface plasmon resonance (LSPR). With advances in nanofabrication techniques, SERS has attracted great attention for label-free molecular sensing and imaging. However, the practical use of SERS has often encountered an inherent issues regarding a molecule transfer step where target molecules need to be within the close proximity of a SERS-active surface by either mixing with nanoparticles or coating onto surface-bound nanostructures. To address this issue, we have developed stamping surface-enhanced Raman spectroscopy (S-SERS) for label-free, multiplexed, molecular sensing and large-area, high-resolution molecular imaging on a flexible, non-plasmonic surface without solution-phase molecule transfer. In this technique, a polydimethylsiloxane (PDMS) thin film and nanoporous gold disk SERS substrate play the roles as molecule carrier and Raman signal enhancer, respectively. After stamping the SERS substrate onto the PDMS film, SERS measurements can be directly taken from the “sandwiched” target molecules. The performance of S-SERS is evaluated by the detection of Rhodamine 6G (R6G), urea, and its mixture with acetaminophen (APAP), in physiologically relevant concentration range, along with corresponding SERS spectroscopic maps. S-SERS features simple sample preparation, low cost, and high reproducibility, which could lead to SERS-based sensing and imaging for point-of-care and forensics applications.

Morphological, plasmonic and SERS characterization of DC-sputtered gold nanoislands

Chemical mapping of molecules adsorbed on plasmonic nanostructures is a powerful technique for biomolecular sensing, surface chemistry and plasmon-matter interactions. DC-sputtered gold nanoisland (GNI) substrates have attracted significant attention recently due to its excellent plasmonic enhancement, structural stability and simple fabrication. We provide multimodal characterization of GNI morphological evolution by correlating data obtained from scanning electron microscopy (SEM), localized surface plasmon resonance (LSPR) extinction spectroscopy and surface-enhanced Raman scattering (SERS) microscopy. A rigorous determination of the SERS enhancement factor for benzenethiol self-assembled monolayers on evolving GNI substrates is presented. Rapid statistical analysis shows excellent large-area SERS uniformity by hyperspectral Raman imaging systems based on active-illumination which enables parsimonious sampling of only 2.7% area of the field of view, greatly improving sampling efficiency.

Monolithically integrated microfluidic nanoporous gold disk (NPGD) surface-enhanced Raman scattering (SERS) sensor for rapid and label-free biomolecular detection

We present a novel microfluidic surface-enhanced Raman scattering (SERS) sensor for rapid and label-free biomolecular detection. Our sensor design mitigates a common limiting factor in microfluidic SERS sensors that utilize integrated nanostructures: low-efficiency transport of biomolecules to nanostructured surface which adversely impacts sensitivity. Our strategy is to increase the total usable nanostructured surface area, which provides more adsorption sites for biomolecules. Specifically, nanoporous gold disk (NPGD) array, a highly effective SERS substrate, has been monolithically integrated inside a microfluidic chip. Individual NPGD is known to feature an order of magnitude larger surface area than its projected disk area. The increased surface area arises from nanoscale pores and ligaments 3- dimensionally distributed in the NPGD, which manifest themselves as high-density SERS hot-spots. High-density NPGD arrays further guarantee large coverage of these hot-spots on the microchannel floor. The SERS-active NPGD arrays enable highly-reproducible SERS measurements with relative intensity variations from 8% to -8%. R6G solutions in the concentrations ranging from 1 μM to 1 mM have been detected and quantitatively evaluated, and the performance of the sensor in continuous-flow condition has been assessed. Moreover, the sensor’s capabilities have been studied by detecting and identifying a physiological metabolite (urea), and the results show lower detection limit compared to best results from most recent work using integrated nanostructured surface inside microchannels. We expect that the sensor would be applicable for detecting, identifying and quantifying molecules for some point-of-care applications, i.e. urine screening.