Buoyancy-Driven Micro/-Nanomotors: From Fundamentals to Applications.

The progression of self-powered micro/-nanomotors (MNMs) has rapidly evolved over the past few decades, showing applications in various fields such as nanotechnology, biomedical engineering, microfluidics, environmental science, and energy harvesting. Miniaturized MNMs transduce chemical/biochemical energies into mechanical motion for navigating through complex fluidic environments with directional control via external forces fields such as magnetic, photonic, and electric stimuli. Among various propulsion mechanisms, buoyancy-driven MNMs have received noteworthy recognition due to their simplicity, efficiency, and versatility. Buoyancy force-driven motors harness the principles of density variation-mediated force to overcome fluidic resistance to navigate through complex environments. Restricting the propulsion in one direction helps to control directional movement, making it more efficient in isotropic solutions. The changes in pH, ionic strength, chemical concentration, solute gradients, or the presence of specific molecules can influence the motion of buoyancy-driven MNMs as evidenced by earlier reports. This review aims to provide a fundamental and detailed analysis of the current state-of-the-art in buoyancy-driven MNMs, aiming to inspire further research and innovation in this promising field.

Nonequilibrium Dynamics of Transient Autoelectrophoresis and Effect of Surface Heterogeneity.

Nonuniform proton flux around a reactive Janus particle as a result of zone selective heterogeneous surface reaction leads to the formation of asymmetric electrical double layers (EDLs) which assists in generating a proximate electric field dipole around the Janus particle to initiate autoelectrophoretic migration. To estimate the force of the autoelectrophoretic motion of such Janus particles, a mathematical model is set up taking Poisson-Nernst-Plank (PNP) equations coupled with the Navier-Stokes (NS) equations with appropriate boundary conditions. To track the actual motion of these particles, we employ moving deforming mesh and fluid-structure interactions (fsi) of COMSOL Multiphysics while a finite element method is deployed for solving the set of modeled equations. At the outset, transient genesis of the electric field around the particle owing to the nonuniform proton flux has been explored. We further explore the detailed unsteady particle dynamics of the autoelectrophoretic motion with the help of fluid structure interaction physics. It has been observed that the concept of perfect ionic equilibrium in autoelectrophoretic motion is hard to achieve. The autoelectrophoretic particle undergoes continuous change in terms of the ionic concentration around it, speed of the particle, and the transient electric field gradient across the particle. The parametric variation of proton flux reveals that at a relatively lower proton flux a quasi-equilibrium state can be achieved, whereas for higher proton flux the phenomenon can be a pure nonequilibrium case. This parametric study has been done to support the transient dynamics. It has also been shown that the presence of chemical heterogeneity on the particle surface can alter the dynamics of the particle significantly, and the chemical heterogeneity can be used as a tool to control directionality and tuning speed of autoelectrophoretic motion.

Electrochemical Energy Harvesting Using Microbial Active Matter.

With the continuous growth in world population and economy, the global energy demand is increasing rapidly. Given that non-renewable energy sources will eventually deplete, there is increasing need for clean, alternative renewable energy sources, which will be inexpensive and involve minimum risk of environmental pollution. In this paper, harnessing the activity of cupric reductase NDH-2 enzyme present in Escherichia coli bacterial cells, we demonstrate a simple and efficient energy harvesting strategy within an electrochemical chamber without the requirement of any external fuels or force fields. The transduction of energy has been demonstrated with various strains of E. coli, indicating that this strategy could, in principle, be applicable for other microbial catalytic systems. We offer a simple mechanism of the energy transduction process considering the bacterial enzyme-mediated redox reaction occurring over the working electrode of the electrochemical cell. Also, the amount of energy generated has been found to be depending on the motility of bacteria within the experimental chamber, suggesting possible opportunities for developing microbial motility-controlled small scale power generators. Finally, we show that the Faradaic electrochemical energy harvested is large enough to power a commercial light emitting diode connected to an amplifier circuit. We expect the present study to generate sufficient interest within soft condensed matter and biophysics communities, and offer useful platforms for controlled energy generation at the small scales.

Point-of-Care Biosensing of Urinary Tract Infections Employing Optoplasmonic Surfaces Embedded with Metal Nanotwins.

We report the synthesis of gold nanotwins (Au NTs) on a solid and transparent glass substrate which in turn has been employed for the selective optoplasmonic detection of Escherichia coli (EC) bacteria in human urine for the point-of-care diagnosis of urinary tract infections (UTIs). As compared to the single nanoparticle systems (Au NPs), the Au NTs show an enriched localized surface plasmon resonance (LSPR) due to the enhancement of the electric field under electromagnetic irradiation, e.g., photon, which helps in improving the limits of detection. For this purpose, initially a simple glass surface has been coated with Au NPs, with the help of the linker 3-aminopropyl-triethoxysilane - APTES. The surface has been linked further with another Au NP with the help of the 1,10-alkane-dithiol linker with two thiol ends, which eventually leads to the development of the optoplasmonic surface with Au NTs and an enhanced LSPR response. Subsequently, the EC specific aptamer has been chemically immobilized on the surface of Au NTs with the blocking of free sites via bovine serum albumin (BSA). Remarkably, Raman spectroscopy unfolds a 7-fold increase in the peak intensities with the Au NTs on the glass surface as compared to the surface coated with isolated Au NPs. The enhancement in the LSPR response of glass substrates coated with Au NTs and the EC specific aptamer has been further utilized for the selective and sensitive detection of UTIs. The results have been verified with the help of UV-visible spectroscopy to establish the utility of the proposed sensing methodology. An extensive interference study with other bacterial species unveils the selectivity and specificity of the proposed optoplasmonic sensors toward EC with a detection range of 5 × 103 to 107 CFU/mL. Intuitively, the method is more versatile in a sense that the sensor can be made specific to any other pathogens by simply changing the design of the aptamer. Finally, a low-cost, portable, and point-of-care optoplasmonic transduction setup is designed with a laser light illumination source, a sample holder, and a sensitive photodetector for the detection of UTIs in human urine.

Pathways to community transmission of COVID-19 due to rapid evaporation of respiratory virulets.

HYPOTHESIS: The formation of virus-laden colloidal respiratory microdroplets - the sneeze or cough virulets and their evaporation driven miniaturization in the open air are found to have a significant impact on the community transmission of COVID-19 pandemic. SIMULATION DETAILS: We simulate the motions and trajectories of virulets by employing laminar fluid flow coupled with droplet tracing physics. A force field analysis has been included considering the gravity, drag, and inertial forces to unleash some of the finer features of virulet trajectories leading to the droplet and airborne transmissions of the virus. Furthermore, an analytical model corroborates temperature (T) and relative humidity (RH) controlled droplet miniaturization. RESULTS: The study elucidates that the tiny (1-50 µm) and intermediate (60-100 µm) size ranged virulets tend to form bioaerosol and facilitate an airborne transmission while the virulets of larger dimensions (300 to 500 µm) are more prone to gravity dominated droplet transmission. Subsequently, the mapping between the T and RH guided miniaturization of virulets with the COVID-19 cases for six different cities across the globe justifies the significant contribution of miniaturization-based bioaerosol formation for community transmission of the pandemic.

Carbon dots and Methylene blue facilitated photometric quantification of Hemoglobin.

Early detection and monitoring of any abnormality of Hemoglobin (Hb) concentration in whole blood samples are important as this may be related to anemia, leukemia, dengue, etc. To facilitate quantitative detection and to monitor the hemoglobin level in the blood, we attempt to develop a low-cost, portable point of care (POC) device based on the spectrophotometric principle. Optical sensitivities of carbon quantum dots (CDs) are found to be highly responsive, while there is a selective reaction between Hb and reduced form of Methylene Blue (MBred). The interaction of Hb, MBred, and CDs is delineated using UV-Visible (UV-Vis) spectroscopy. CDs have a characteristic UV-Vis peak at âˆ¼ 347 nm, and it shows a gradual increase in intensity with a slight red shift (∼355 nm) on the progressive increase in Hb concentration. Simultaneously, the colorless MBred is oxidized to its blue oxidized form MBox and its characteristic peak starts reappearing at âˆ¼ 663 nm. These responses are exploited to quantify Hb concentration with a limit of detection (LOD) as low as âˆ¼ 2 g dL-1 in a developed POC device, and the results are validated with the clinical data obtained from a local hospital with reasonably good agreement. This photometric detection approach can be adopted for other quantitative biosensors.

Oxygen Generation Using Catalytic Nano/Micromotors.

Gaseous oxygen plays a vital role in driving the metabolism of living organisms and has multiple agricultural, medical, and technological applications. Different methods have been discovered to produce oxygen, including plants, oxygen concentrators and catalytic reactions. However, many such approaches are relatively expensive, involve challenges, complexities in post-production processes or generate undesired reaction products. Catalytic oxygen generation using hydrogen peroxide is one of the simplest and cleanest methods to produce oxygen in the required quantities. Chemically powered micro/nanomotors, capable of self-propulsion in liquid media, offer convenient and economic platforms for on-the-fly generation of gaseous oxygen on demand. Micromotors have opened up opportunities for controlled oxygen generation and transport under complex conditions, critical medical diagnostics and therapy. Mobile oxygen micro-carriers help better understand the energy transduction efficiencies of micro/nanoscopic active matter by careful selection of catalytic materials, fuel compositions and concentrations, catalyst surface curvatures and catalytic particle size, which opens avenues for controllable oxygen release on the level of a single catalytic microreactor. This review discusses various micro/nanomotor systems capable of functioning as mobile oxygen generators while highlighting their features, efficiencies and application potentials in different fields.

Noninvasive Point-of-Care Nanobiosensing of Cervical Cancer as an Auxiliary to Pap-Smear Test.

A potential cancer antigen (Ag), protein-phosphatase-1-gamma-2 (PP1γ2), with a restricted expression in testis and sperms has been identified as a biomarker specific to cervical cancer (CaCx). Detection of this cancer biomarker antigen (NCB-Ag) in human urine opens up the possibility of noninvasive detection of CaCx to supplement the dreaded and invasive Pap-smear test. A colorimetric response of an assembly of gold nanoparticles (Au NPs) has been employed for the quantitative, noninvasive, and point-of-care-testing of CaCx in the urine. In order to fabricate the immunosensor, Au NPs of sizes ∼5-20 nm have been chemically modified with a linker, 3,3'-di-thio-di-propionic-acid-di(n-hydroxy-succinimide-ester) (DTSP) to attach the antibody (Ab) specific to the NCB-Ag. Interestingly, the addition of Ag to the composite of Ab-DTSP-Au NPs leads to a significant hypsochromic shift due to a localized surface plasmon resonance phenomenon, which originates from the specific epitope-paratope interaction between the NCB-Ag and Ab-DTSP-Au NPs. The variations in the absorbance and wavelength shift during such attachments of different concentrations of NCB-Ag on the Ab-DTSP-Au NPs composite have been employed as a calibration to identify NCB-Ag in human urine. An in-house prototype has been assembled by integrating a light-emitting diode of a narrow range wavelength in one side of a cuvette in which the reaction has been performed while a sensitive photodetector to the other side to transduce the transmitted signal associated with the loading of NCB-Ag in the Ab-DTSP-Au NPs composite. The proposed immunosensing platform has been tested against other standard proteins to ensure noninterference alongside proving the proof-for-specificity of the NCB detection.

Proton exchange membrane and bio-Fenton micro fuel cells for energy harvesting, gas leakage detection, and dye degradation.

The present work focuses on the non-conventional design and operation of micro fuel cells. Two different kinds of fuel cells, Proton Exchange Membrane (PEM) and Biological Fenton (BF) based fuel cells, are fabricated to harvest energy. For the PEM fuel cell, H2 and O2 are generated by Mg/HCl reaction and Fenton's reaction respectively, and are subsequently fed into two terminals of the PEM fuel cell. For the BF fuel cell, the reaction product of hemoglobin (Hb) with hydrogen peroxide (H2O2) is used as a source of chemical fuel to generate electrical energy within the fuel cell. An array of PEM microscale fuel cells is fabricated to scale up the reaction which can be used for MEMS/NEMS applications. Furthermore, the application of this adhesive and flexible PEM fuel cell as a hydrogen leakage sensor is demonstrated. In the BF fuel cell, an electronic imbalance across a carbon tape is generated owing to the formation of reactive hydroxyl radicals and concurrent electrons in the system. The generation of a highly oxidizing hydroxyl radical is also utilized to degrade Methylene Blue (MB) dye along with energy harvesting. This multi-purpose fuel cell can be synergistically used in industrial applications of waste treatment as well as energy production.

Multimodal chemo-/magneto-/phototaxis of 3G CNT-bots to power fuel cells.

We report the development of a 3G microswimmer, namely, CNT-bot, capable of undergoing acid-, alkali-, magneto- and phototaxis inside acidic or alkaline baths of peroxide fuel and/or water. The use of carboxyl-functionalised multi-walled carbon nanotubes (MWCNTs) facilitated the propulsion of CNT-bots in an alkaline-water solution by ejecting carbon-dioxide bubbles. Furthermore, doping of magnetite nanoparticles (FeONPs), ferrous ions (Fe2+) and titanium dioxide nanoparticles (TiONPs) induces magnetic, chemical and photonic modes of propulsion. While FeONPs stimulated magnetotaxis at a rate of up to ~10 body lengths per second under the influence of a bar magnet, chemotaxis of a similar speed in a peroxide fuel was achieved by bubble-propulsion of oxygen gas originating from the Fenton reaction. In addition, the light-stimulated photo-Fenton reaction led to phototaxis of CNT-bots. A thin coating of magnesium imparted a half-faced Janus appearance to the CNT-bots, which facilitated motion in normal or acidic water media through the ejection of hydrogen gas bubbles. This chemotaxis could be transformed into pH-stimulated directional motion by establishing an acid or alkali concentration gradient across the peroxide and/or water baths. The capacity of CNT-bots to produce oxygen (hydrogen) bubbles in peroxide (acidic water) fuel was exploited to power a PEM fuel cell to generate electricity. The pure oxygen and hydrogen gases generated by CNT-bots in separate chambers were fed directly into the fuel cell in which the incessant motions of the particle facilitated the creation and release of the pure gases to achieve on-demand electricity generation. The motor could also induce dye degradation through advanced oxidation owing to the production of intermediate hydroxyl radicals during the Fenton reaction.

Electric field assisted multicomponent reaction in a microfluidic reactor for superior conversion and yield.

We explore the improvements in yield and conversion of a chemical reaction inside a two-phase microfluidic reactor when subjected to an externally applied alternating current (AC) electric field. A computational fluid dynamic (CFD) framework has been developed to incorporate the descriptions of the two-phase flow, multicomponent transport and reaction, and the Maxwell's stresses generated at oil-water interface owing to the presence of the externally applied electric field. The CFD model ensures that the reactants are flown into a microchannel together with the oil and water phases before the reaction takes place at the interface and products diffuse back to the bulk phases. The study unveils that the variation in the intensity of the AC field helps in converting a two-phase stratified flow into an oil-in-water microemulsion composed of oil slugs, plugs, or droplets. Importantly, the results also suggest that harnessing the vortices inside or outside these flow patterns helps in the improvement in mass transfer across the interface, which can be employed to improve the yield and conversion of a reaction. We have shown an example case of a pseudo-first order reaction for which the variation in frequency and intensity of AC field is found to form higher surface-to-volume-ratio flow patterns having a higher throughput. The convective recirculation in and around these miniaturized flow morphologies increase the rate of mass transfer, mixing of reactant and products, conversion of reactant, and yield of products. The results reported can be of significance in the design and development of future advanced-flow rector technologies.

Pattern-Directed Ordering of Spin-Dewetted Liquid Crystal Micro- or Nanodroplets as Pixelated Light Reflectors and Locomotives.

Chemical pattern directed spin-dewetting of a macroscopic droplet composed of a dilute organic solution of liquid crystal (LC) formed an ordered array of micro- and nanoscale LC droplets. Controlled evaporation of the spin-dewetted droplets through vacuum drying could further miniaturize the size to the level of ∼90 nm. The size, periodicity, and spacing of these mesoscale droplets could be tuned with the variations in the initial loading of LC in the organic solution, the strength of the centripetal force on the droplet, and the duration of the evaporation. A simple theoretical model was developed to predict the spacing between the spin-dewetted droplets. The patterned LC droplets showed a reversible phase transition from nematic to isotropic and vice versa with the periodic exposure of a solvent vapor and its removal. A similar phase transition behavior was also observed with the periodic increase or reduction of temperature, suggesting their usefulness as vapor or temperature sensors. Interestingly, when the spin-dewetted droplets were confined between a pair of electrodes and an external electric field was applied, the droplets situated at the hydrophobic patches showed light-reflecting properties under the polarization microscopy highlighting their importance in the development of micro- or nanoscale LC displays. The digitized LC droplets, which were stationary otherwise, showed dielectrophoretic locomotion under the guidance of the external electric field beyond a threshold intensity of the field. Remarkably, the motion of these droplets could be restricted to the hydrophilic zones, which were confined between the hydrophobic patches of the chemically patterned surface. The findings could significantly contribute in the development of futuristic vapor or temperature sensors, light reflectors, and self-propellers using the micro- or nanoscale digitized LC droplets.