Fusheng Zhao

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.

Simultaneous Chemical and Refractive Index Sensing in the 1-2.5 μm Near-Infrared Wavelength Range on Nanoporous Gold Disks.

Near-infrared (NIR) absorption spectroscopy provides molecular and chemical information based on overtones and combination bands of the fundamental vibrational modes in the infrared wavelengths. However, the sensitivity of NIR absorption measurement is limited by the generally weak absorption and the relatively poor detector performance compared to other wavelength ranges. To overcome these barriers, we have developed a novel technique to simultaneously obtain chemical and refractive index sensing in 1-2.5 μm NIR wavelength range on nanoporous gold (NPG) disks, which feature high-density plasmonic hot-spots of localized electric field enhancement. For the first time, surface-enhanced near-infrared absorption (SENIRA) spectroscopy has been demonstrated for high sensitivity chemical detection. With a self-assembled monolayer (SAM) of octadecanethiol (ODT), an enhancement factor (EF) of up to ∼10(4) has been demonstrated for the first C-H combination band at 2400 nm using NPG disk with 600 nm diameter. Together with localized surface plasmon resonance (LSPR) extinction spectroscopy, simultaneous sensing of sample refractive index has been achieved for the first time. The performance of this technique has been evaluated using various hydrocarbon compounds and crude oil samples.

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.

Photothermal inactivation of heat-resistant bacteria on nanoporous gold disk arrays

A rapid photothermal bacterial inactivation technique has been developed by irradiating near-infrared (NIR) light onto bacterial cells (Escherichia coli, Bacillus subtilis, Exiguobacterium sp. AT1B) deposited on surfaces coated with a dense, random array of nanoporous gold disks (NPGDs). With the use of cell viability tests and SEM imaging results, the complete inactivation of the pathogenic and heat-resistant bacterial model strains is confirmed within ~25 s of irradiation of the NPGD substrate. In addition to irradiation control experiments to prove the efficacy of the bacterial inactivation, thermographic imaging showed an immediate averaged temperature rise above 200 °C within the irradiation spot of the NPGD substrate. The light-gated photothermal effects on the NPGD substrate offers potential applications for antimicrobial and nanotherapeutic devices due to strong light absorption in the tissue optical window, i.e., the NIR wavelengths, and robust morphological structure that can withstand high instantaneous thermal shocks.

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.

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.