Xinyuan Chong

Detecting explosive molecules from nanoliter solution: A new paradigm of SERS sensing on hydrophilic photonic crystal biosilica.

We demonstrate a photonic crystal biosilica surface-enhanced Raman scattering (SERS) substrate based on a diatom frustule with in-situ synthesized silver nanoparticles (Ag NPs) to detect explosive molecules from nanoliter (nL) solution. By integrating high density Ag NPs inside the nanopores of diatom biosilica, which is not achievable by traditional self-assembly techniques, we obtained ultra-high SERS sensitivity due to dual enhancement mechanisms. First, the hybrid plasmonic-photonic crystal biosilica with three dimensional morphologies was obtained by electroless-deposited Ag seeds at nanometer sized diatom frustule surface, which provides high density hot spots as well as strongly coupled optical resonances with the photonic crystal structure of diatom frustules. Second, we discovered that the evaporation-driven microscopic flow combined with the strong hydrophilic surface of diatom frustules is capable of concentrating the analyte molecules, which offers a simple yet effective mechanism to accelerate the mass transport into the SERS substrate. Using the inkjet printing technology, we are able to deliver multiple 100pico-liter (pL) volume droplets with pinpoint accuracy into a single diatom frustule with dimension around 30µm×7µm×5µm, which allows for label-free detection of explosive molecules such as trinitrotoluene (TNT) down to 10-10M in concentration and 2.7×10-15g in mass from 120nL solution. Our research illustrates a new paradigm of SERS sensing to detect trace level of chemical compounds from minimum volume of analyte using nature created photonic crystal biosilica materials.

Plasmonics-enhanced metal–organic framework nanoporous films for highly sensitive near-infrared absorption

Combined plasmonic nanocrystals and metal–organic framework thin-films are fabricated for sensing gases in the near-infrared range. This nanocomposite thin-film shows a highly sensitive response in near-infrared absorption, which is attributed to preconcentration of gas molecules in metal–organic framework pores causing close proximity to the electromagnetic fields at the plasmonic nanocrystal surface.

Ultrashort Near-Infrared Fiber-Optic Sensors for Carbon Dioxide Detection

In this paper, we report a fiber-optic carbon dioxide (CO2) near-infrared (IR) absorption sensor with only 8-cm sensing length that is coated with nanoporous metalorganic framework material Cu-BTC (BTC = benzene-1,3, 5-tricarboxylate). The multimode optical fiber was etched by hydrofluoric acid to remove the cladding and part of the core, resulting in larger evanescent field to sense the near-IR absorption induced by the adsorbed CO2. The Cu-BTC thin film with 100 nm thickness was then grown onto the ethced core through a stepwise layer-by-layer method. Our real-time measurement results show that the CO2 detection limit is better than 500 ppm and the overall response time is 40 s for absorption and 75 s for desorption. To the best of our knowledge, this is the shortest near-IR fiber-optic sensor for CO2 detection at 1.57-μm wavelength.

Effect of finite metallic grating size on Rayleigh anomaly-surface plasmon polariton resonances.

Rayleigh anomalies (RAs) and surface plasmon polaritons (SPPs) on subwavelength metallic gratings play pivotal roles in many interesting phenomena such as extraordinary optical transmission. In this work, we present a theoretical analysis of the effect of finite metallic grating size on RA-SPP resonances based on the combination of rigorous coupled wave analysis and finite aperture diffraction. One-dimensional arrays of gold subwavelength gratings with different device sizes were fabricated and the optical transmission spectra were measured. As the grating size shrinks, the broadening of the RA-SPP resonances is predicted by the theoretical model. For the first order RA-SPP resonances, the results from this model are in good agreement with the spectra measured from the fabricated plasmonic gratings.

Broadband on-chip near-infrared spectroscopy based on a plasmonic grating filter array

We demonstrate an ultra-compact, broadband on-chip near-infrared (NIR) spectroscopy system based on a narrow-band plasmonic filter array. The entire filter array, consisting of 28 individual subwavelength metallic gratings, was monolithically integrated in a thin gold film on a quartz substrate, covering a 270 nm spectra from 1510 nm to 1780 nm. In order to achieve a high spectral resolution, extremely narrow slits are created for the gratings with a polymer waveguide layer on top, generating narrow-band guided-mode resonances through coupling with the surface-plasmon resonances of the metallic gratings. Experimental results show that the transmission bands of the filter array have full width at half-maximum of only 7 nm-13 nm, which is sufficient for NIR spectroscopy. The NIR absorption spectroscopy of xylene using the on-chip plasmonic filter array matches very well with the results from conventional Fourier transform infrared spectroscopy, which proves the great potential for NIR sensing applications.

Guided-Mode Resonance Grating with Self-Assembled Silver Nanoparticles for Surface-Enhanced Raman Scattering Spectroscopy

We designed and fabricated guided-mode resonance (GMR) gratings on indium-tin-oxide (ITO) thin film to generate a significantly enhanced local electric field for surface-enhanced Raman scattering (SERS) spectroscopy. Ag nanoparticles (NPs) were self-assembled onto the surface of the grating, which can provide a large amount of “hot-spots” for SERS sensing. The ITO gratings also exhibit excellent tolerance to fabrication deviations due to the large refractive index contrast of the ITO grating. Quantitative experimental results of 5,5’-dithiobis(2-nitrobenzoic acid) (DTNB) demonstrate the best enhancement factor of ~14× on ITO gratings when compared with Ag NPs on a flat ITO film, and the limit of detection (LOD) of DTNB is as low as 10 pM.

Microfluidic diatomite analytical devices for illicit drug sensing with ppb-Level sensitivity

The escalating research interests in porous media microfluidics, such as microfluidic paper-based analytical devices, have fostered a new spectrum of biomedical devices for point-of-care (POC) diagnosis and biosensing. In this paper, we report microfluidic diatomite analytical devices (μDADs), which consist of highly porous photonic crystal biosilica channels, as an innovative lab-on-a-chip platform to detect illicit drugs. The μDADs in this work are fabricated by spin-coating and tape-stripping diatomaceous earth on regular glass slides with cross section of 400×30µm2. As the most unique feature, our μDADs can simultaneously perform on-chip chromatography to separate small molecules from complex biofluidic samples and acquire the surface-enhanced Raman scattering spectra of the target chemicals with high specificity. Owing to the ultra-small dimension of the diatomite microfluidic channels and the photonic crystal effect from the fossilized diatom frustules, we demonstrate unprecedented sensitivity down to part-per-billion (ppb) level when detecting pyrene (1ppb) from mixed sample with Raman dye and cocaine (10 ppb) from human plasma. This pioneering work proves the exclusive advantage of μDADs as emerging microfluidic devices for chemical and biomedical sensing, especially for POC drug screening.

Plasmonic nanopatch array with integrated metal–organic framework for enhanced infrared absorption gas sensing

In this letter, we present a nanophotonic device consisting of plasmonic nanopatch array (NPA) with integrated metal-organic framework (MOF) for enhanced infrared absorption gas sensing. By designing a gold NPA on a sapphire substrate, we are able to achieve enhanced optical field that spatially overlaps with the MOF layer, which can adsorb carbon dioxide (CO2) with high capacity. Experimental results show that this hybrid plasmonic-MOF device can effectively increase the infrared absorption path of on-chip gas sensors by more than 1100-fold. The demonstration of infrared absorption spectroscopy of CO2 using the hybrid plasmonic-MOF device proves a promising strategy for future on-chip gas sensing with ultra-compact size.

Ultra-sensitive lab-on-a-chip detection of Sudan I in food using plasmonics-enhanced diatomaceous thin film

Sudan I is a carcinogenic compound containing an azo group that has been illegally utilized as an adulterant in food products to impart a bright red color to foods. In this paper, we develop a facile lab-on-a-chip device for instant, ultra-sensitive detection of Sudan I from real food samples using plasmonics-enhanced diatomaceous thin film, which can simultaneously perform on-chip separation using thin layer chromatography (TLC) and highly specific sensing using surface-enhanced Raman scattering (SERS) spectroscopy. Diatomite is a kind of nature-created photonic crystal biosilica with periodic pores and was used both as the stationary phase of the TLC plate and photonic crystals to enhance the SERS sensitivity. The on-chip chromatography capability of the TLC plate was verified by isolating Sudan I in a mixture solution containing Rhodamine 6G, while SERS sensing was achieved by spraying gold colloidal nanoparticles into the sensing spot. Such plasmonics-enhanced diatomaceous film can effectively detect Sudan I with more than 10 times improvement of the Raman signal intensity than commercial silica gel TLC plates. We applied this lab-on-a-chip device for real food samples and successfully detected Sudan I in chili sauce and chili oil down to 1 ppm, or 0.5 ng/spot. This on-chip TLC-SERS biosensor based on diatomite biosilica can function as a cost-effective, ultra-sensitive, and reliable technology for screening Sudan I and many other illicit ingredients to enhance food safety.

On-chip near-infrared spectroscopy of CO2 using high resolution plasmonic filter array

We report an ultra-compact, cost-effective on-chip near-infrared spectroscopy system for CO2 sensing using narrow-band optical filter array based on plasmonic gratings with a waveguide layer. By varying the periodicity of the gratings, the transmission spectra of the filters can be continuously tuned to cover the 2.0 μm sensing window with high spectral resolution around 10 nm. Our experimental results show that the on-chip spectroscopy system can resolve the two symmetric vibrational bands of CO2 at 2.0 μm wavelength, which proves its potential to replace the expensive commercial IR spectroscopy system for on-site gas sensing.