Junfeng Pan

Floating assembly of diatom Coscinodiscus sp. microshells

Diatoms have silica frustules with transparent and delicate micro/nano scale structures, two dimensional pore arrays, and large surface areas. Although, the diatom cells of Coscinodiscus sp. live underwater, we found that their valves can float on water and assemble together. Experiments show that the convex shape and the 40 nm sieve pores of the valves allow them to float on water, and that the buoyancy and the micro-range attractive forces cause the valves to assemble together at the highest point of water. As measured by AFM calibrated glass needles fixed in manipulator, the buoyancy force on a single floating valve may reach up to 10 μN in water. Turning the valves over, enlarging the sieve pores, reducing the surface tension of water, or vacuum pumping may cause the floating valves to sink. After the water has evaporated, the floating valves remained in their assembled state and formed a monolayer film. The bonded diatom monolayer may be valuable in studies on diatom based optical devices, biosensors, solar cells, and batteries, to better use the optical and adsorption properties of frustules. The floating assembly phenomenon can also be used as a self-assembly method for fabricating monolayer of circular plates.

Biosilica structures obtained from Nitzschia , Ditylum , Skeletonema , and Coscinodiscus diatom by a filtration-aided acid cleaning method

A filtration-aided acid cleaning method was used to collect biosilica structures from a diatom culture medium, natural seawater, or water bloom. Cell extraction, acid cleaning, and acid removal were all performed on a polytetrafluoroethylene (PTFE) filter cloth, significantly improving the treatment capacity and efficiency of the traditional acid wash method. Five typical diatoms were cultivated in the laboratory for acid cleaning. Different growth speeds were introduced, and different process parameters for acid cleaning were utilized. After the acid cleaning, biosilica structures were collected from the frustules of diatoms using different methods. Girdle bands and valves of Coscinodiscus sp. were separated by floating of the valves. Central spines of Ditylum brightwellii and valves of Skeletonema costatum were separately collected by settling or filtration. Rod-like frustules, such as those of Bacillaris paradoxa, are not suitable for large quantities of acid wash. The silica structures were observed and tested using an AFM-calibrated glass needle to determine their elasticity. Elasticity tests showed that ringent girdle bands are more flexible than complete ones (Coscinodiscus sp.) and that both long-chain clusters of Nitzschia palea and central spines of D. brightwellii have certain elasticities. The required pressure for deforming or breaking the biosilica structures of diatoms was also determined.

Key factors influencing the optical detection of biomolecules by their evaporative assembly on diatom frustules

Diatoms have silica frustules with transparent and delicate micro/nano scale structures, multilevel pore arrays, and large specific surface areas. We explored the potential of diatom frustules as biomolecule support for use in optical detection, for example, in protein or DNA biochips and “lab-on-a-chip” sensors. After the solution was evaporated, most particles in the solution assembled on the frustules. Experiments indicated that this phenomenon occurs because of the large specific surface of the frustules; consequently, we studied the capacity of frustules to increase the density of antibodies. The frustules of diatoms Coscinodiscus sp., Navicula sp., and Nitzschia palea were used in this study. The colored particles for optical detection included standard protein, soybean lecithin, bovine serum albumin, and human immunoglobulin G labeled with fluorescein and carbonic black ink. The results showed that the fluorescein isothiocyanate protein was densely assembled on the frustules and exhibited a fluorescence signal that is 2.5 times stronger than that of glass. Compared with the traditional glass substrate, the frustules significantly improved the antibody density and detection signals. The evaporating assembly method was used for measuring the load capacity of frustules for different antibodies; this method can be used to quantitatively bind two or more antibodies to the frustule, which may be valuable in lab-on-a-chip sensors. The design scheme of high-throughput diatom-based biochips was discussed. Through analysis, we hypothesized that diatom frustules with a large specific surface area, high transparency and pore permeability, small sizes and heights, and flat surfaces are particularly suitable for optical detection of biomolecules.

Hydrofluoric acid-assisted bonding of diatoms with SiO2-based substrates for microsystem application

Diatom, with delicate three-dimensional porous structures and texture, has a promising application in micro-nanotechnology especially biosensing. In order to achieve a diatom-based compound substrate, a fabrication technique is developed for hydrofluoric acid (HF) bonding of diatom with SiO2-based substrate at a temperature as low as 80 °C. The bonding conditions are optimized with various HF concentrations and applied pressure. The optimized HF concentration is found to be in the range of 0.8% to 1.2% and applied pressure is from 0.4.0 MPa to 0.6.0 MPa. The morphological integrity and nano-microscale substructures of the diatoms after bonding are characterized. The bonding strength is approximately 0.435 MPa.

Multi-layer hierarchical array fabricated with diatom frustules for highly sensitive bio-detection applications

Diatoms have delicate porous structures which are very beneficial in improving the absorbing ability in the bio-detection field. In this study, multi-layered hierarchical arrays were fabricated by packing Nitzschia soratensis (N. soratensis) frustules into Cosinodiscus argus (C. argus) frustules to achieve advanced sensitivity in bio-detection chips. Photolithographic patterning was used to obtain N. soratensis frustule arrays, and the floating behavior of C. argus frustules was employed to control their postures for packing N. soratensis frustule array spots. The morphology of the multi-layer C. argus–N. soratensis package array was investigated by scanning electron microscopy, demonstrating that the overall and sub-structures of the diatom frustules were retained. The signal enhancing effect of multi-layer C. argus–N. soratensis packages was demonstrated by fluorescent antibody test results. The mechanism of the enhancement was also analyzed, indicating that both complex hierarchical frustule structures and optimized posture of C. argus frustules were important for improving bio-detection sensitivities. The technique for fabricating multi-layer diatom frustules arrays is also useful for making multi-functional biochips and controllable drug delivery systems.

Bonding of diatom frustules and Si substrates assisted by hydrofluoric acid

Diatoms, with hierarchical micro/nanoscale porous silica structures, have promising application in micro-nanotechnology especially biochemical sensing. In order to explore their potential and prepare diatom based substrates for biochemical sensor application, a fabrication technology for bonding diatom frustules and Si substrates was developed. The bonding process was carried out at 75 °C and assisted by hydrofluoric acid (HF). The bonding mechanism was discussed and several bonding conditions were adjusted to keep the morphological integrity of diatom frustules after bonding. The bonding pressure was optimized from 2.0 × 104 Pa to 3.0 × 104 Pa and the HF concentration from 0.4% to 0.6%. And the optimal shear bonding strength achieved was 0.72 MPa. In addition, bonded diatom frustules were further used as masks to obtain nano gold pillar arrays for surface-enhanced Raman scattering (SERS) detection.

Biopattern transfer using diatom frustules for fabrication of functional micro/nanostructures

Abstract. Diatom frustules exhibit various sophisticated shapes with highly ordered hierarchical porous nanostructures, which are promising for applications in the biomimetic fabrication of nanostructured materials. We propose a universal biopattern transfer process for the fabrication of functional micro/nanostructures using diatom frustules as the biotemplates. Porous silicon microcylinders with a thickness of 20  μm are fabricated by deep reactive ion etching of a silicon substrate, which is covered by a layer of diatom frustules. With a similar process, a fast atom beam technique is used to etch the silicon substrate and silicon nanolattices are obtained. By depositing a thin layer of gold film on the diatom bonded silicon substrate, followed by releasing the diatom frustules by diluted HF, gold nanodisks with a thickness of 30 nm are successfully fabricated. The nanodisk array arranges in diamond or radial patterns, replicating the nanostructure of diatom frustules. In addition, a parylene nanodot array is also demonstrated using this diatom-based biopattern transfer process.

Diatom based biosensor for high sensitive fluorescence detection based on a spin-on glass bonding technique

Diatom, with delicate nanoscale porous SiO2 structures, has a promising application in biosensing. We bond diatoms with SiO2 substrate using spin-on glass (SOG) as interlayer to acquire a high performance diatom based fluorescence detecting biosensor. As diatoms can improve proteins density, diatom based biosensors have higher sensitivity compared with traditional protein sensors. This biosensor is fully made of SiO2 which can be modified for biomass attachment. Nanoscale structures of bonded diatoms are better preserved than any related works. The highest bonding strength is 0.6 MPa. Human immunoglobulin G (IgG) is attached on traditional protein sensors and diatom based biosensors as antibody. After hybridized with Cy3-goat anti-human IgG, the diatom based biosensor provides a fluorescence detection signal 2.5 times higher than the traditional one. Given improved fluorescence detecting sensitivity, this new kind of diatom based biosensor can be widely used for various protein detections.