(7ag) Novel Biosensors for Transformative Healthcare

Accurate, fast and low-cost detection of biomarkers is essential for effective prescreening of many chronic diseases and cancers. Conventional biosensors such as DNA microarray and real time PCR are golden standards for identification and quantification of nucleic acids. Although very sensitive, these platforms rely on (i) accurate duplication and (ii) fluorescence labeling of target molecules, which are challenging for the case of small RNAs. Ligand bias has been found in high throughput screening of microRNAs. For other types of biomarkers such as proteins, amino acids and peptides, conventional detection methods such as ELISA or Western Blot also rely on fluorescence labeling and do not achieve desirable detection limits and fast throughput. Furthermore, for novel biomarkers such as exosomes, which contain genetic information, no standard quantification methods have been developed. Therefore, novel biosensors that employ new sensing schemes are required especially for biomarkers such as microRNAs, proteins and exosomes.

Nanoplasmonics involves surface electromagnetic waves confined to a nano-scale region (hot spot) sustained by a plasmon resonator, thus allowing sensitive biomolecule sensing without the need for amplification. During my PhD studies, I developed a microRNA-screening platform that is based on a nano-cone array. A low detection limit, down to about 100 microRNAs per pixel, was achieved due to enhanced fluorescence by nanoplasmonic modes localized at the tip of the cones (Wang et al, Biomicrofluidics 8 021101, 2014; Wang et al, Optics Express 21 6609, 2013; Wang et al AIChE J. 59 1830, 2013). This platform is ideal for multiplexing. Detection of multiple targets in a panel can be achieved by addressing each fiber core optically and using photochemistry to selectively attach different probes. Another sensing platform that I co-developed, based on nanocapillaries, achieved similar detection limit. In this platform, plasmonic hotspots are formed by packing gold nanoparticles inside tips of a nanocapillary (Liu et al, Biomicrofluidics 7 061102, 2014). Hot spots in between nanoparticles can be dynamically tuned by applying external bias to achieve maximum enhancement factors. This platform overcomes the diffusion limit by electrically driving microRNAs to the detection region and achieves femto-molar concentration detection within 15 minutes.

Although providing improved sensing performance, the above-mentioned nanoplasmonic platforms still rely on labeling of target molecules with high efficiency dyes. This additional step does not only increase cost, but also affects the efficiency and yield, especially for small RNAs, proteins and amino acids. UV (Ultra-Violet) plasmonics is a young research field yet holds promise in enabling label-free biosensing. Although the native fluorescence of biomolecules resides in the UV range of the spectrum, the quantum yield of the native fluorescence is low when compared to the visible fluorescence of dyes. To improve the detection limit, I have been exploring UV plasmonics in order to modify the native fluorescence emission properties of biomolecules. In this regard, during my postdoc training at the University of Utah, I have investigated, both experimentally and numerically, novel UV plasmonic materials and geometries for the purpose of enhancing native fluorescence emission from biomolecules, in particular, amino acids. Experimentally, ultrafast fluorescence spectroscopy is used to study the change of emission rate of native fluorescence of biomolecules. Novel UV materials that I studied include aluminum (Al), magnesium (Mg), gallium (Ga) and their alloys. Geometries studied include nanoapertures, bowtie antennas and bulls eye apertures. Aluminum provides the best performance in terms of fluorescence enhancement, while Mg is a better candidate at shortening the lifetime of biomolecules (Wang et al, JPCC 121 11650, 2017). Moreover, Al bowtie antennas have been found to achieve the highest radiative emission rate enhancement, up to 40 times, based on numerical simulations (Lotubai et al, SPIE optics and Photonics, 2017). With improved emission rate, more photons are collected from each molecule, thus improving the detection limit.

Besides detection limit, multiplexing is another important functionality for biosensing platforms as multiple biomarkers might be required to identify certain pathogens. Active devices that change their responses corresponding to different biomarkers can be employed for multiplexing. For this purpose I am developing a tunable UV plasmonic device based on the interaction of pi-plasmons in graphene with surface (and localized) plasmon modes in aluminum plasmonic structures. In this regard, I recently demonstrated that by employing graphene with different doping concentrations, the plasmonic resonance of an Al hole-array can be shifted (Wang et al, CLEO, 2017). This is the first demonstration of graphene in a tunable UV device and paves the way for other tunable UV plasmonic devices that can be used for active biosensing.

Future research directions that I intend to undertake include:

1. UV-plasmonics-enabled label-free nucleic-acid/protein sensor. My research goal is to realize an integrated sensing platform that can achieve real-time detection of biomolecules based on their intrinsic fluorescence with detection limit comparable to conventional fluorescence assays. I will apply my knowledge on UV plasmonic materials and nanostructures both in experiments and simulations to design such platforms. The changes of fluorescent intensity, lifetime or diffusion coefficients upon binding of target molecules with probe molecules can be employed to detect and quantify the number of target molecules. UV plasmonic structures will be used in such platforms to achieve comparable sensing limits as conventional fluorescence assays. This platform will integrate sample pretreatment, electro-kinetic manipulation of molecules and detection of biomolecules in one chip and will be able to detect nucleic acids, proteins, or amino acids.
2. Exosome identification and quantification for early cancer diagnosis. Cell-derived extracellular vesicles such as exosomes are of increasing interest for understanding intercellular communication, diagnosis, and curing of cancer. One of the challenges in early cancer diagnosis is being able to detect a small amount of exosomes derived from cancer cells from a vast number of exosomes originating from healthy cells. Therefore, developing a method to rapidly identify and isolate cancer-derived exosomes from healthy exosomes is crucial for early cancer diagnosis. My goal is to develop a label-free exosome flow cytometer that can rapidly identify and sort exosomes based on their surface protein contents, which carry the signatures of their cellular origin. After sorting of exosomes, genetic analysis will be carried out to identify the type and stage of cancer using the microRNA screening platform that I developed during my early work.
3. Investigation of novel plasmonic devices based on 2D materials in the UV and far-IR ranges. Spectral signatures of biomolecules such as their native fluorescence lie in the UV range whereas vibrational and rotational signatures lie at the other end of the spectrum, i.e. the far-IR. In spite of this, because of its technological maturity and the easy availability of efficient materials and devices, current research is mainly performed at the visible range rather than at the UV or far-IR. From this perspective, my research interest is to investigate novel materials at these un-conventional frequency ranges and to use these new materials for biomedical applications. With this motivation in mind, I recently began exploring the plasmonic properties of structures composed of 2D materials, e.g. graphene. Graphene has been shown to have good affinity to biomolecules and also very strong UV resonances due to pi-plasmons, thus an optical sensor integrated with graphene might be able to deliver improved sensitivity. One of my current research projects, which I independently initiated while at the University of Utah, involves designing a tunable plasmonic UV biosensor using graphene. Moreover, due to its unconventional band-structure, graphene has been shown by other authors to exhibit strong plasmonic resonances at the mid and far-IR ranges. In general, studying the interaction of 2D materials with biomolecules at these “unconventional” frequency ranges, where both the native spectral features of molecules and the strongest optical response of these materials takes place is an exciting new research direction that I foresee to be highly transformative in the field.

Teaching interests:

I have taught as a teaching assistant for Biomedical Engineering Transport Phenomena' and 'Mathematical Methods for Engineers' and guest lectured for ‘Electrokinetics’, 'Applied Electromagnetics' and 'Nanophotonics'. I will be interested in teaching core Chemical Engineering courses such as 'Transport phenomena', 'Thermodynamics', 'Mathematical Methods for Engineers', 'Fluid mechanics', etc. and also teach specialized course such as 'Biosensing', 'Nanotechnology', 'Nanophotonics', ‘Electrokinetics’, 'Micro and Nano fabrication' and so on.