Research Interests

of the Sailor Group at UC San Diego

Our group is expert in the chemistry, electrochemistry, and optical properties of silicon-based nanomaterial systems. Our research focuses on silicon nanotechnology, with emphasis on drug delivery materials, in vivo and in vitro imaging with photoluminescent silicon quantum dots, photonic crystals, remote chemical sensing, biosensing, and catalysis. The major research themes are given below.

Porous Silicon Nanoparticles for Delivery of Biologics

nanoparticles image

We are developing approaches to deliver protein and nucleic acid-based therapeutics for treatment of infectious diseases, diseases of the retina, and cancer. We developed a new chemistry to load oligonucleotides and other negatively charged payloads into porous Si nanoparticles, based on precipitation of endogenous silicate with calcium or magnesium ions (Adv. Mater. 2016, 28, 7962). This achieved the highest mass loading of nucleic acid for any nanoparticle system to date, and it is the enabling feature of the porous Si nanoparticle systems we have been developing for treatment of cancer, inflammatory diseases, infections, and neurologic injuries. This included the first example of siRNA used to reprogram the immune system's macrophages to cure a lethal bacterial infection (Nat. Commun. 2018, 9, 1969). This work has also made a commercial impact as inventions related to these porous Si particles are being translated to the clinic).

Silicon “Nano-Cages” as Protective Hosts for Enzyme Catalysis

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Using porous silicon-based nanopaticles to encapsulate enzymes while retaining their ability to function is a strong theme in our lab. We use porous silicon nanoparticles (pSiNPs) as enzyme cages, utilizing the aqueous chemistry of silicon to dynamically restructure the mesopore structure, immobilizing and confining the enzyme. With the proper trapping chemistry, enzyme stability and performance can be substantially improved, even in challenging environments that result in complete or near-complete loss of activity for the free enzyme. These nanocomposites have potential as cellular probes and as vehicles for enzyme replacement therapy (Chem. Mater. 2023, 35, 10247).

Non-Toxic, Biocompatible Silicon “Quantum Dots” as Imaging Agents

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We demonstrated the first in vivo use of luminescent porous Si as an imaging agent (Nature Mater. 2009, 8, 331). Our most highly cited paper to date (> 1700 citations), in 2012 it was highlighted by the editors of Nature Materials as one of the “Landmark Papers” published by the Journal in the previous 10 years. We provided the first example of time-gated imaging in live animals using porous Si nanoparticles (Nat. Commun. 2013, 4, 2326), where we showed that interference from tissue autofluorescence and other competing fluorophores can be reduced by more than 100-fold. This work has played a major role in the current explosion of interest in silicon quantum dots as in vitro and in vivo probes. Our work here is enabled by various aqueous-based surface oxidation chemistries being developed to generate highly emissive, but biodegradable Si-SiO2 core-shell nanomaterials (ACS Nano 2015, 9, 6233).

Polymer Composites with Porous Silicon Particles

image showing porous silicon nanoparticles embedded in PLGA nanofibers

Poly Lactic-co-Glycolic Acid (PLGA) is one example of several FDA-approved polymers that are biodegradable, physically robust, and highly biocompatible. These polymers have been extensively studied as delivery vehicles for small molecule drugs, and to a lesser extent biologics (such as protein and nucleic acid therapeutics). The ability to load and control release of sensitive biologics in biocompatible polymers is an ongoing challenge, mainly due to incompatibilities with the solvents used in processing the polymers, to incompatibilities of the biologic with the host polymer matrix, and to low solubility of the biologic in the polymer. We proposed one potential solution to this problem (Adv. Mater. 2018, 30, 1706785), which involves first incorporating the therapeutic into porous silicon nanoparticles and then incorporating the particles into polymer nanofibers. We are now deploying functional examples of this approach in various application areas. For example, we have shown that molecules currently of high interest for neuronal repair (such as a small molecule PTEN inhibitor, a TrkB aptamer, and the protein nerve growth factor NGF), can enhance neuronal growth when incorporated into the hybrid porous silicon/polymer nanofiber scaffolds. This approach also allows tuning of the temporal release profile of the APIs. In the particular example above and shown in the image, nanofibers on the order of a few hundred nanometers were shown to significantly enhance neuron growth (Zuidema, J. M. et al., "Porous Silicon Nanoparticles Embedded in Poly(lactic-co-glycolic acid) Nanofiber Scaffolds Deliver Neurotrophic Payloads to Enhance Neuronal Growth. Adv. Func. Mater. 2020, 30, 2002560).

Microparticles of Porous Silicon

Smart Dust

Since their discovery in 1992 (Science 1992, 255, 66), particles made from porous silicon have been of interest to a wide range of research disciplines. We deploy micron-scale photonic crystals of porous Si (so-called “Smart Dust” Proc. Nat. Acad. Sci. 2003, 100, 10607), in applications such as environmental and remote sensing. The fundamental electrochemistry involved in anodization of silicon allows the generation of complex nanostructures. The substantial impact of the work includes commercial development of the “spectral barcodes” concept as non-toxic tags for pharmaceuticals and durable goods (TruTags, inc).

Chemical Sensors and Biosensors using Porous Silicon Optical Films

This work traces its history to the 1990s, when porous silicon was shown to be a useful platform for chemical and biological sensing based on thin film optical interferometry (J. Electrochem. Soc. 1993, 140, 3492; Science 1997, 278, 840). The sensors detect the refractive index of the analyte, and specificity is achieved by using a capture probe (such as an antibody) embedded in the high surface area porous silicon matrix. Our current biosensing strategies use the RIFTS (Reflective Interferometric Fourier Transform Spectroscopy) method (J. Am. Chem. Soc. 2006, 128, 4250), which has been widely adopted in the chemical and biochemical sensing communities, and SLIM (Spectroscopic Liquid Infiltration Method), commonly used to quantify thickness and porosity of porous optical films (Adv. Funct. Mater. 2007, 17, 1153).

Surface Chemistry of Silicon Nanomaterials

We have a longstanding interest in harnessing the chemistry of silicon to enable the various applications of silicon nanostructures. In collaboration with Barry Arkles and his team at Gelest, we developed catalyst-free dehydrocoupling reactions to modify porous Si and porous silicon oxide surfaces (Angew. Chem. 2016, 128, 6533), which have been very useful in stabilizing and imparting chemical functionality to the material. It proceeds at substantially lower temperatures and is less susceptible to water impurities than the more commonly employed hydrosilylation reaction. We also have been developing heterocyclic silanes as simple, rapid, and one-pot reagents to modify silicon surfaces--these reagents are an attractive alternative to the ubiquitous tri- or mono-alkoxysilane coupling reagents currently used to modify hydroxy-terminated surfaces (J. Am. Chem. Soc. 2016, 138, 15106).

Summer School for Silicon Nanotechnology (SSSiN)

All trainees in the Sailor Group begin with the SSSiN, an immersive two-month workshop on the preparation, characterization, and applications of porous silicon-based nanomaterials. This provides the basic training of incoming graduate students, undergraduates, visiting scholars, and high school students.

Although the scope of our work at the University is limited to basic research into the fundamental properties of materials, many of the concepts developed in our labs have been translated to products and processes in the commercial world. Any commercial products and processes that emerge from our fundamental discoveries are outside the course and scope of our University-related employment; however, we actively encourage transition of technologies to the commercial sector with the assistance of UCSD’s Office of Innovation and Commercialization