Photocaged functional nucleic acids for spatiotemporal imaging in biology

Imaging of species in living organisms with high spatiotemporal resolution is essential for understanding biological processes. While functional nucleic acids (FNAs), such as catalytic nucleic acids and aptamers, have emerged as effective sensors for a wide range of molecules, photocaged control of these FNAs has played a key role in translating them into bioimaging agents with high spatiotemporal control. In this review, we summarize methods and results of photocaged FNAs based on photolabile modifications, photoisomerization, and photothermal activation. Future directions, including strategies to improve the performance of these photocaged FNAs, are also described.     

Fabrication of Carbon Nanotube High-Frequency Nanoelectronic Biosensor for Sensing in High Ionic Strength Solutions

The unique electronic properties and high surface-to-volume ratios of single-walled carbon nanotubes (SWNT) and semiconductor nanowires (NW) ^1-4 make them good candidates for high sensitivity biosensors. When a charged molecule binds to such a sensor surface, it alters the carrier density⁵ in the sensor, resulting in changes in its DC conductance. However, in an ionic solution a charged surface also attracts counter-ions from the solution, forming an electrical double layer (EDL). This EDL effectively screens off the charge, and in physiologically relevant conditions ~100 millimolar (mM), the characteristic charge screening length (Debye length) is less than a nanometer (nm). Thus, in high ionic strength solutions, charge based (DC) detection is fundamentally impeded^6-8.We overcome charge screening effects by detecting molecular dipoles rather than charges at high frequency, by operating carbon nanotube field effect transistors as high frequency mixers^9-11. At high frequencies, the AC drive force can no longer overcome the solution drag and the ions in solution do not have sufficient time to form the EDL. Further, frequency mixing technique allows us to operate at frequencies high enough to overcome ionic screening, and yet detect the sensing signals at lower frequencies^11-12. Also, the high transconductance of SWNT transistors provides an internal gain for the sensing signal, which obviates the need for external signal amplifier.Here, we describe the protocol to (a) fabricate SWNT transistors, (b) functionalize biomolecules to the nanotube¹³, (c) design and stamp a poly-dimethylsiloxane (PDMS) micro-fluidic chamber¹⁴ onto the device, and (d) carry out high frequency sensing in different ionic strength solutions¹¹.     

Electrospun cellulose acetate nanofibers: The present status and gamut of biotechnological applications

Cellulose acetate (CA) has been a material of choice for spectrum of utilities across different domains ranging from high absorbing diapers to membrane filters. Electrospinning has conferred a whole new perspective to polymeric materials including CA in the context of multifarious applications across myriad of niches. In the present review, we try to bring out the recent trend (focused over last five years' progress) of research on electrospun CA fibers of nanoscale regime in the context of developmental strategies of their blends and nanocomposites for advanced applications. In the realm of biotechnology, electrospun CA fibers have found applications in biomolecule immobilization, tissue engineering, bio-sensing, nutraceutical delivery, bioseparation, crop protection, bioremediation and in the development of anti-counterfeiting and pH sensitive material, photocatalytic self-cleaning textile, temperature-adaptable fabric, and antimicrobial mats, amongst others. The present review discusses these diverse applications of electrospun CA nanofibers.     

Recent progress in metal-based molecular probes for optical bioimaging and biosensing

Biological imaging and biosensing from subcellular/cellular level to whole body have enabled non-invasive visualisation of molecular events during various biological and pathological processes, giving great contributions to the rapid and impressive advances in chemical biology, drug discovery, disease diagnosis and prognosis. Optical imaging features a series of merits, including convenience, high resolution, good sensitivity, low cost and the absence of ionizing radiation. Among different luminescent probes, metal-based molecules offer unique promise in optical bioimaging and biosensing in vitro and in vivo, arising from their small sizes, strong luminescence, large Stokes shifts, long lifetimes, high photostability and tunable toxicity. In this review, we aim to highlight the design of metal-based molecular probes from the standpoint of synthetic chemistry in the last 2 years for optical imaging, covering d-block transition metal and lanthanide complexes and multimodal imaging agents.     

Sensing cellular biochemistry with fluorescent chemical–genetic hybrids

Fluorescent biosensors are powerful tools for the detection of biochemical events inside cells with high spatiotemporal resolution. Biosensors based on fluorescent proteins often suffer from issues with photostability and brightness. On the other hand, hybrid, chemical–genetic systems present unique opportunities to combine the strengths of synthetic, organic chemistry with biological macromolecules to generate exquisitely tailored semisynthetic sensors.     

Kinetic analysis of nanoparticle‐protein interactions using a plasmon waveguide resonance

A plasmon waveguide resonance (PWR) sensor is proposed for studying the interaction between gold nanoparticles and proteins. The ability of the PWR sensor to operate in both TM and TE Polarizations, i.e. its polarization diversity, facilitates the simultaneous spectroscopy of the nanoparticles surface reactions using both polarizations. The response of each polarization to streptavidin‐biotin binding at the surface of gold nanoparticles is investigated in real time. Finally, using the principles of multimode spectroscopy, the nanoparticle's surface reactions are decoupled from the bulk solution refractive index variations.

Schematic diagram of the NP‐modified PWR sensor     

A polymer nanostructured Fabry-Perot interferometer based biosensor

A polymer nanostructured Fabry–Perot interferometer (FPI) based biosensor is reported. Different from a conventional FPI, the nanostructured FPI has a layer of Au-coated nanopores inside its cavity. The Au-coated nanostructure layer offers significant enhancement of optical transducing signals due to the localized surface plasmon resonance effect and also due to the significantly increased sensing surface area, which is up to at least two orders of magnitude larger than that of a conventional FPI-based biosensor. Using this technical platform, the immobilization of captures proteins (protein A) on the nanostructure layer and their binding with immunoglobulin G (IgG) has been monitored in real time, resulting in the shift of the interference fringes of the optical transducing signals. Current results show that the limit-of-detection of the biosensor should be lower than 10 pg/mL for IgG-protein A binding.     

Towards biological applications of nanophotonic tweezers

Optical trapping (synonymous with optical tweezers) has become a core biophysical technique widely used for interrogating fundamental biological processes on size scales ranging from the single-molecule to the cellular level. Recent advances in nanotechnology have led to the development of ‘nanophotonic tweezers,’ an exciting new class of ‘on-chip’ optical traps. Here, we describe how nanophotonic tweezers are making optical trap technology more broadly accessible and bringing unique biosensing and manipulation capabilities to biological applications of optical trapping.     

Gold nanoparticle-based signal amplification for biosensing

Colloidal gold nanoparticles (AuNPs), with unique properties such as highly resonant particle plasmons, direct visualization of single nanoclusters by scattering of light, catalytic size enhancement by silver deposition, conductivity, and electrochemical properties, are very attractive materials for several applications in biotechnology. Furthermore, as excellent biological tags, AuNPs can be easily conjugated with biomolecules and retain the biochemical activity of the tagged biomolecules, making AuNPs ideal transducers for several biorecognition applications. The goal of this article is to review recent advances of using AuNPs as labels for signal amplification in biosensing applications. We focus on the signal amplification strategies of AuNPs in biosensing/biorecognition, more specifically, on the main optical and electrochemical detection methods that involve AuNP-based biosensing. Particular attention is given to recent advances and trends in sensing applications.     

Recent progresses in biomedical applications of aptamer-functionalized systems

Aptamers, known as “chemical antibodies” are screened via a combinational technology of systematic evolution of ligands by exponential enrichment (SELEX). Due to their specific targeting ability, high binding affinity, low immunogenicity and easy modification, aptamer-functionalized systems have been extensively applied in various fields and exhibit favorable results. However, there is still a long way for them to be commercialized, and few aptamer-functionalized systems have yet successfully entered clinical and industrial use. Thus, it is necessary to overview the recent research progresses of aptamer-functionalized systems for the researchers to improve or design novel and better aptamer-functionalized systems. In this review, we first introduce the recent progresses of aptamer-functionalized systems’ applications in biosensing, targeted drug delivery, gene therapy and cancer cell imaging, followed by a discussion of the challenges faced with extensive applications of aptamer-functionalized systems and speculation of the future prospects of them.