Optical biosensing has achieved remarkable levels of sensitivity and has enabled early detection of various toxins and biomarkers. Fluorescence spectroscopy is among the most common and powerful optical detection techniques, capable of single molecule detection. This is done by exciting the sample using a light source, collecting the fluorescence light inherent in the sample or on a reporter molecule, and measuring the fluorescence spectrum using a spectrometer. This modality is effective for multiplex sensing as full spectral data is acquired. However, fluorescence spectroscopy requires multiple measurements at multiple points to achieve a representative sampling of a sensor. Fluorescence imaging is a detection modality similar to fluorescence spectroscopy, but replaces the spectrometer with an imager such as a camera thus reducing cost and complexity. Imaging allows data acquisition at multiple points in a large area of your sensor in a single measurement making it a more efficient sensing method but does not acquire spectral data. Both fluorescence sensing modalities have been shown to be very powerful in pristine laboratory settings but when the equipment or measurement area are not ideal, additional enhancement is needed. This can be achieved by implementing a sensing substrate capable of enhancing fluorescence signals to practical detection levels. Diatoms are unicellular marine organisms that grow a biosilica shell called a frustule. These frustules are porous with nanostructured patterns and represent naturally occurring photonic crystals which are known to enhance excitation and emission of fluorophores. In addition to the optical enhancements of diatoms, the large surface area allows for large numbers of analytes to aggregate making fluorescence signals stronger. In this work, we employ naturally occurring photonic crystal diatoms to create a sensor capable of enhancing the fluorescence of a standard sandwich immunoassay. Using this sensor, we achieved detection down to 10-16 M using fluorescence spectroscopy and 10-15 M for fluorescence imaging. These represent a 100× and 10× enhancement for the two respective detection modalities over equivalent, non-diatom sensors. This highlights the capability of our sensor to enhance fluorescence optical signals and its potential to be used in point-of-care biosensing applications.
Optical biosensing is a growing area of research known for its low limits of detection. Among optical sensing techniques, fluorescence detection is among the most established and prevalent. Fluorescence imaging is an optical biosensing modality that exploits the sensitivity of fluorescence in an easy-to-use process. Fluorescence imaging allows a user to place a sample on a sensor and use an imager, such as a camera, to collect the results. The image can then be processed to determine the presence of the analyte. Fluorescence imaging is appealing because it can be performed with as little as a light source, a camera and a data processor thus being ideal for nontrained personnel without any expensive equipment. Fluorescence imaging sensors generally employ an immunoassay procedure to selectively trap analytes such as antigens or antibodies. When the analyte is present, the sensor fluoresces thus transducing the chemical reaction into an optical signal capable of imaging. Enhancement of this fluorescence leads to an enhancement in the detection capabilities of the sensor. Diatoms are unicellular algae with a biosilica shell called a frustule. The frustule is porous with periodic nanopores making them biological photonic crystals. Additionally, the porous nature of the frustule allows for large surface area capable of multiple analyte binding sites. In this paper, we fabricate a diatom based ultra-sensitive fluorescence imaging biosensor capable of detecting the antibody mouse immunoglobulin down to a concentration of 1 nM. The measured signal has an enhancement of 6× when compared to sensors fabricated without diatoms.
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.
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.
We present low-cost bioenabled surface-enhanced Raman scattering (SERS) substrates that can be massively produced in sustainable and eco-friendly methods with significant commercial potentials for the detection of food contamination and drinking water pollution. The sensors are based on diatom frustules with integrated plasmonic nanoparticles. The ultra-high sensitivity of the SERS substrates comes from the coupling between the diatom frustules and Ag nanoparticles to achieve dramatically increased local optical field to enhance the light-matter interactions for SERS sensing. We successfully applied the bioenabled SERS substrates to detect melamine in milk and aromatic compounds in water with sensitivity down to 1μg/L.
Diatoms are single-celled algaes that make photonic-crystal-like silica shells or frustules with hierarchical micro- and
nano-scale features consisting of two-dimensional periodic pores. In this paper, we present an innovative label-free
optical sensor based on a biological-plasmonic hybrid nanostructure by self-assembling silver (Ag) nanoparticles into
diatom frustules. The photonic-crystal-like diatom frustules provide a spatially confined electric field with enhanced
intensity that can form hybrid photonic-plasmonic modes through the optical coupling with Ag nanoparticles. The
experimental results demonstrate 4-6x and 9-12x improvement of sensitivities to detect the Raman dye for resonance
and nonresonance SERS sensing, respectively.
Dye-sensitized solar cells (DSSCs) rely on a network of titanium dioxide nanoparticles for electron transport and must balance carrier generation and collection. Adding photonic structures may increase light capture without affecting carrier collection. Diatoms are single-celled algae that biologically fabricate silicon dioxide cell walls which resemble photonic crystal slabs. We present a simple fabrication strategy that allows for uniform and controlled placement of biosilica within DSSCs. Integration of biosilica reduces photoanode transmittance to less than 5% prior to dye sensitization at loading levels as low as 6 wt% biosilica. Increased biosilica loading (17 wt%) provides additional enhancements in photocurrent generation. Reflectance measurements suggest that the enhancement results from the combined effects of photonic resonance and Mie scattering. Overall efficiency of these devices is improved by 8% and 14%, respectively.
Diatoms are a group of single-celled photosynthetic algae that make skeletal shells of hydrated amorphous silica, called
frustules, which possess hierarchical nanoscale photonic crystal features made by a bottom-up approach at ambient
temperature and pressure. In this paper, we theoretically investigate electric field enhancements of plasmonic
nanoparticles coated on the surface of diatom skeletal shells. Surface-Enhanced Raman Scattering substrates are
prepared by evaporating 10 nm thick silver film and self-assembling silver nanoparticles on diatom surfaces, which show
significantly better SERS signals than silver nanoparticles on flat glass substrates.
Diatoms are single-celled photosynthetic algae that make silica shells or "frustules" with intricate features patterned at
the nano and microscales. In this study, antibody-functionalized diatom biosilica frustules serve as a biosensor platform
for selective and label free antibody-antigen immunocomplex formation by enhanced photoluminescence. Biosilica
frustules of 10 micron diameter were isolated from cells of the centric marine diatom Cyclotella sp. They were then
mounted on glass and covalently functionalized with the model antibody Rabbit Immunoglobulin G (IgG) to yield a
uniform nanostructured surface that selectively binds to its complimentary antigen, Goat anti-Rabbit IgG. Diatom
frustules possess an intrinsic capacity to emit blue light when excited with a UV laser light source, a property called
photoluminescence. Binding the antibody-functionalized diatom frustule with its complimentary antigen selectively
enhanced the intrinsic photoluminescence intensity of the diatom frustule by a factor of three, whereas challenging the
antibody-functionalized diatom frustule with a non-complimentary antigen, Goat anti-human IgG did not change the
intrinsic photoluminescence intensity. The nucleophilic immunocomplex increases the photoluminescence by donating
electrons to non-radiative sites on the photoluminescent diatom biosilica, thereby decreasing non-radiative electron
decay and increasing radiative emission. The intensified photoluminescence intensity is correlated to the antigen, goat
anti-rabbit IgG concentration, with a binding constant of 2.8 ± 0.7x10-7 M.
Diatoms are single-celled algae which possess silica shells called "frustules" that contain periodic submicron scale
features. A diatom cell culture process was used to fabricate a two-dimensional photonic crystal slab of Ge-doped
biosilica that possessed 120 nm holes, 330 nm lattice constant, and dielectric constant of 8.5. This material was
integrated into an electroluminescent (EL) device by spin coating of the frustules onto indium tin oxide, followed by
atomic layer deposition of 400 nm hafnium silicate. No photonic band gap was predicted. However, the EL spectrum
possessed resonant UV line emissions that were consistent with photonic band calculations. An EL band gap between
500-640 nm was also observed between blue and red EL line emissions. These EL characteristics have not been observed
previously, and are unique to the diatom photonic crystal. This study represents a first step towards the realization of
optoelectronic devices which utilize nanoscale components fabricated through cell culture.
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