Paul LeDuff

Optofluidic sensing from inkjet-printed droplets: the enormous enhancement by evaporation-induced spontaneous flow on photonic crystal biosilica

Novel transducers for detecting an ultra-small volume of an analyte solution play pivotal roles in many applications such as chemical analysis, environmental protection and biomedical diagnosis. Recent advances in optofluidics offer tremendous opportunities for analyzing miniature amounts of samples with high detection sensitivity. In this work, we demonstrate enormous enhancement factors (106-107) of the detection limit for optofluidic analysis from inkjet-printed droplets by evaporation-induced spontaneous flow on photonic crystal biosilica when compared with conventional surface-enhanced Raman scattering (SERS) sensing using the pipette dispensing technology. Our computational fluid dynamics simulation has shown a strong recirculation flow inside the 100 picoliter droplet during the evaporation process due to the thermal Marangoni effect. The combination of the evaporation-induced spontaneous flow in micron-sized droplets and the highly hydrophilic photonic crystal biosilica is capable of providing a strong convection flow to combat the reverse diffusion force, resulting in a higher concentration of the analyte molecules at the diatom surface. In the meanwhile, high density hot-spots provided by the strongly coupled plasmonic nanoparticles with photonic crystal biosilica under a 1.5 μm laser spot are verified by finite-difference time domain simulation, which is crucial for SERS sensing. Using a drop-on-demand inkjet device to dispense multiple 100 picoliter analyte droplets with pinpoint accuracy, we achieved the single molecule detection of Rhodamine 6G and label-free sensing of 4.5 × 10-17 g trinitrotoluene from only 200 nanoliter solution.

Micro‐photoluminescence of single living diatom cells

Diatoms are single-celled microalgae that possess a nanostructured, porous biosilica shell called a frustule. This study characterized the micro-photoluminescence (μ-PL) emission of single living cells of the photosynthetic marine diatom Thalassiosira pseudonana in response to UV laser irradiation at 325 nm using a confocal Raman microscope. The photoluminescence (PL) spectrum had two primary peaks, one centered at 500-510 nm, which was attributed to the frustule biosilica, and a second peak at 680 nm, which was attributed to auto-fluorescence of photosynthetic pigments. The portion of the μ-PL emission spectrum associated with biosilica frustule in the single living diatom cell was similar to that from single biosilica frustules isolated from these diatom cells. The PL emission by the biosilica frustule in the living cell emerged only after cells were cultivated to silicon depletion. The discovery of the discovery of PL emission by the frustule biosilica within a single living diatom itself, not just its isolated frustule, opens up future possibilities for living biosensor applications, where the interaction of diatom cells with other molecules can be probed by μ-PL spectroscopy. Copyright

SERS sensing of sub-nanoliter analyte on diatom biosilica using inkjet printing

Diatoms are unicellular algae which have photonic-crystal-like biosilica frustules consisting of many pores. Each diatom frustule has a dimension around 10~20μm and can be used as a miniaturized biosensor. In this article, we demonstrate surface-enhanced Raman scattering (SERS) sensing of sub-nanoliter analyte on diatom biosilica with self-assembled silver nanoparticles (Ag NPs). An inkjet printer is used to dispense multiple ~100 pico-liter volume analyte droplets with pinpoint accuracy and precision onto each individual diatom frustule. Experimental results show up to 3x higher SERS signals of R6G on diatom compared with those from conventional colloidal SERS substrates. Furthermore, down to 10-14M R6G detection ability was also demonstrated through the inkjet printing strategy.

Chemical and Biological Sensing Using Diatom Photonic Crystal Biosilica With In-Situ Growth Plasmonic Nanoparticles

In this paper, we described a new type of bioenabled nano-plasmonic sensors based on diatom photonic crystal biosilica with in-situ growth silver nanoparticles and demonstrated label-free chemical and biological sensing based on surface-enhanced Raman scattering (SERs) from complex samples. Diatoms are photosynthetic marine micro-organisms that create their own skeletal shells of hydrated amorphous silica, called frustules, which possess photonic crystal-like hierarchical micro- & nanoscale periodic pores. Our research shows that such hybrid plasmonic-biosilica nanostructures formed by cost-effective and eco-friendly bottom-up processes can achieve ultra-high limit of detection for medical applications, food sensing, water/air quality monitoring and geological/space research. The enhanced sensitivity comes from the optical coupling of the guided-mode resonance of the diatom frustules and the localized surface plasmons of the silver nanoparticles. Additionally, the nanoporous, ultra-hydrophilic diatom biosilica with large surface-to-volume ratio can concentrate more analyte molecules to the surface of the SERS substrates, which can help to detect biomolecules that cannot be easily adsorbed by metallic nanoparticles.

Photonic crystal enhanced fluorescence immunoassay on diatom biosilica

Fluorescence biosensing is one of the most established biosensing methods, particularly fluorescence spectroscopy and microscopy. These are two highly sensitive techniques but require high-grade electronics and optics to achieve the desired sensitivity. Efforts have been made to implement these methods using consumer grade electronics and simple optical setups for applications such as point-of-care diagnostics, but the sensitivity inherently suffers. Sensing substrates, capable of enhancing fluorescence are thus needed to achieve high sensitivity for such applications. In this paper, we demonstrate a photonic crystal-enhanced fluorescence immunoassay biosensor using diatom biosilica, which consists of silica frustules with sub-100 nm periodic pores. Utilizing the enhanced local optical field, the Purcell effect and increased surface area from the diatom photonic crystals, we create ultrasensitive immunoassay biosensors that can significantly enhance fluorescence spectroscopy as well as fluorescence imaging. Using standard antibody-antigen-labeled antibody immunoassay protocol, we experimentally achieved 100× and 10× better detection limit with fluorescence spectroscopy and fluorescence imaging respectively. The limit of detection of the mouse IgG goes down to 10-16 M (14 fg/mL) and 10-15 M (140 fg/mL) for the two respective detection modalities, virtually sensing a single mouse IgG molecule on each diatom frustule. The effectively enhanced fluorescence imaging in conjunction with the simple hot-spot counting analysis method used in this paper proves the great potential of diatom fluorescence immunoassay for point-of-care biosensing.

Dual-mode immunoassay using photonic crystal biosilica

We demonstrate an ultra-sensitive immunoassay biosensor through photonic crystal enhanced fluorescence and surface-enhanced Raman scattering (SERS) using diatom biosilica. We experimentally achieved enhanced detection limit down to 10⁻¹⁶M and 10⁻¹³M respectively.

Photonic crystal biosilica for biosensing with single molecule sensitivity

Summary form only given. The detection of a small number of molecules in miniature amount of solution is of pivotal significance in many applications including biomedicine, homeland security, forensics, and environmental protection. Surface-Enhanced Raman scattering (SERS) offers the potential for label-free sensing of many chemicals due to its unprecedented sensitivity and specificity. However, the performance of SERS sensing is limited by the plasmonic-active substrates that can provide a high density of hot-spots. We demonstrate an innovative SERS substrates based on photonic crystal biosilica with in-situ synthesized siliver nanoparticles (Ag NPs). The three-dimensional hybrid plasmonic-biosilica morphology was obtained by electroless-deposited Ag seeds at the nanoporous diatom surface, which provided high density hot spots for SERS as well as strongly coupled optical resonances with the photonic crystal structure of diatom frustules. We also discovered that the evaporation-induced microscopic flow combined with the strong hydrophilic surface of diatom frustule 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 pico-liter volume droplets with pinpoint accuracy into a single diatom frustule, achieving label-free detection of explosive molecules (trinitrotoluene) down to 2.7×10-15 grams, and single molecule detection sensitivity of R6G. Our research illustrates a new paradigm of SERS sensing to detect trace level of chemical compounds from minimum volume of analyte using nature created nanophotonic materials.

Dual-mode bioenabled nano-plasmonic sensors for biological and chemical detection

Plasmonic biosensors have greatly overcome the limitations of conventional optical sensors in terms of sensitivity, tunability, photo-stability, and in vivo applicability. In this paper, we present plasmonic biosensors using bioenabled nanomaterials diatom biosilica, with active surface functionalities as affordable and eco-friendly integration platforms of Ag nanoparticles for label-free detection of biomolecules. Dual-mode plasmon sensing mechanisms, including surface-enhanced Raman scattering (SERS) and refractive-index (RI) sensing will be simultaneously implemented on the plasmonic-biosilica nanostructures to obtain quantitative biosensing with structural resolution of the biomolecules. We have achieved ultra-sensitive detection of Rhodamine 6G (R6G) at concentrations as low as 10−10 M. Furthermore, this substrate was used to detect TNT, illustrating the potential application as viable substrates for monitoring pollutant and toxics in environment.