Preeti S. Saxena

Study of mechanism of enhanced antibacterial activity by green synthesis of silver nanoparticles

The extensive use of silver nanoparticles needs a synthesis process that is greener without compromising their properties. The present study describes a novel green synthesis of silver nanoparticles using Guava (Psidium guajava) leaf extract. In order to compare with the conventionally synthesized ones, we also prepared Ag-NPs by chemical reduction. Their optical and morphological characteristics were thoroughly investigated and tested for their antibacterial properties on Escherichia coli. The green synthesized silver nanoparticles showed better antibacterial properties than their chemical counterparts even though there was not much difference between their morphologies. Fourier transform infrared (FTIR) spectroscopic analysis of the used extract and as-synthesized silver nanoparticles suggests the possible reduction of Ag(+) by the water-soluble ingredients of the guava leaf like tannins, eugenol and flavonoids. The possible reaction mechanism for the reduction of Ag(+) has been proposed and discussed. The time-dependent electron micrographs and the simulation studies indicated that a physical interaction between the silver nanoparticles and the bacterial cell membrane may be responsible for this effect. Based on the findings, it seems very reasonable to believe that this greener way of synthesizing silver nanoparticles is not just an environmentally viable technique but it also opens up scope to improve their antibacterial properties.

Cefuroxime axetil loaded solid lipid nanoparticles for enhanced activity against S. aureus biofilm

The present research work is focused on the development of solid lipid nanoparticles of cefuroxime axetil (CA-SLN) for its enhanced inhibitory activity against Staphylococcus aureus produced biofilm. CA-SLN was prepared by solvent emulsification/evaporation method using single lipid (stearic acid (SA)) and binary lipids (SA and tristearin (TS)). Process variables such as volume of dispersion medium, concentration of surfactant, homogenization speed and time were optimized. The prepared SLN were characterized for encapsulation efficiency, drug polymer interaction studies (DSC and FT-IR), shape and surface morphology (SEM and AFM), in vitro drug release, stability studies and in vitro anti biofilm activity against S. aureus biofilm. Among the process variables, increased volume of dispersion medium, homogenization speed and time led to increase in particle size whereas increase in surfactant concentration decreased the particle size. SLN prepared using binary lipids exhibited higher entrapment efficiency than the single lipid. DSC and FT-IR studies showed no incompatible interaction between drug and excipients. CA-SLN showed two folds higher anti-biofilm activity in vitro than pristine CA against S. aureus biofilm.

Facile, rapid and upscaled synthesis of green luminescent functional graphene quantum dots for bioimaging

We report here the upscaled synthesis of green luminescent functionalized graphene quantum dots (FGQDs) by using an inexpensive and commonly occurring natural precursor viz. graphite powder. We observed in our sample that photoluminescence increases for excitation wavelengths of 300 nm to 350 nm and then decreases when excited at 375 to 425 nm for FGQDs at neutral pH. We found that the synthesized FGQDs do not show a drastic change in emission properties when kept under different pH conditions, which makes them a potential candidate for in vivo imaging, where the pH of the culture media plays a crucial role in the maintenance of the fluorescence. Water solubility, and excellent photostability along with low cytotoxicity of FGQDs are manifested as a remarkable bioimaging material.

Nanostructured palladium-reduced graphene oxide platform for high sensitive, label free detection of a cancer biomarker

We report the results of studies related to the fabrication of a palladium nanoparticle decorated-reduced graphene oxide (Pd@rGO) based electrochemical immunosensor for the label free ultrasensitive detection of the prostate-specific antigen (PSA), a prostate cancer biomarker. The synergistic electrochemical activities of Pd and rGO result in an enhanced electron transfer used for the development of an ultrasensitive immunosensor. A facile approach was developed for the in situ synthesis of Pd@rGO using ascorbic acid as the reducing agent which enables the simultaneous reduction of both Pd+2 and GO into Pd nanoparticles and rGO, respectively. XRD, FTIR, SEM and TEM investigations were carried out to characterize the Pd@rGO material. A thin film of nanostructured Pd@rGO was electrophoretically deposited on an ITO coated glass electrode that was subsequently functionalized with anti-PSA antibodies. The electrochemical sensing results of the proposed immunosensor showed a high sensitivity {28.96 μA ml ng−1 cm−2}. The immunosensor is able to detect PSA at concentrations as low as 10 pg ml-1. The simple fabrication method, high sensitivity, good reproducibility and long term stability with acceptable accuracy in human serum samples are the main advantages of this immunosensor.

Biofunctional magnetic nanotube probe for recognition and separation of specific bacteria from a mixed culture

This study highlights the synthesis of an antibody conjugated magnetic carbon nanotube bioprobe for the recognition and separation of Pseudomonas aeruginosa (P. aeruginosa), a gram negative bacterium, from its mixed culture with Staphylococcus aureus (S. aureus). Multiwalled carbon nanotubes containing iron oxide nanoparticles (magnetic carbon nanotubes) were synthesized in a single step by a spray pyrolysis method. The synthesized magnetic nanotubes were characterized by X-ray diffraction, electron microscopy and magnetic property measurements. A P. aeruginosa specific rhodamine-labelled goat anti-Pseudomonas antibody was covalently attached to the magnetic carbon nanotubes to develop a bioprobe. Raman and Fourier transform spectroscopy studies were carried out to confirm the attachment of the antibodies to the magnetic nanotubes. The designed bioprobe was employed for the capture and subsequent separation of P. aeruginosa from its mixed culture with S. aureus. The probing efficiency of the developed bioprobe was characterized and confirmed by culturing the captured P. aeruginosa in selective media followed by fluorescence and scanning electron microscopy studies. A time dependent increase in the capture efficiency of the bioprobe for P. aeruginosa was noticed and found to be 65% within five minutes of incubation. Thus, the designed bioprobe presents a simple, reliable and cost effective diagnostic tool for rapid identification and separation of a particular bacterium from a site of co-infection which is of immense clinical relevance.

MWCNTs as reinforcing agent to the Hap–Gel nanocomposite for artificial bone grafting

The essence of this investigation is to explore MWCNTs as reinforcing agents to strengthen Hap-Gel nanocomposites for artificial bone grafting applications without significantly compromising their biocompatibility. Hap-Gelatin composites, reinforced with various proportions of MWCNTs, were synthesized to optimize the MWCNT content in the composites which yield commendable improvement in the strength. The morphological studies reveal that the MWCNTs act as templates for nucleation of Hap crystals. The biocompatibility of MWCNT reinforced Hap-Gelatin composites were evaluated in animal model through the histopathological investigation of tissues from skin, kidney, and liver. On histopathological examination, no noticeable alteration due to toxicity was found for lower concentration of MWCNTs. Mild reversible changes in the liver and tubular damage in kidney have been observed for higher concentration (4 wt % of MWCNTs). It can be inferred from the findings that MWCNTs, in proportions less than 4%, can successfully be used to reinforce the Hap-Gel nanocomposite to improve its mechanical properties. However, how safe would these CNT reinforced bone implants would be when used for prolonged period in actual physiological conditions needs to be investigated further.

Partially reduced graphene oxide-gold nanorods composite based bioelectrode of improved sensing performance

The present work proposes partially reduced graphene oxide-gold nanorods supported by chitosan (CH-prGO-AuNRs) as a potential bioelectrode material for enhanced glucose sensing. Developed on ITO substrate by immobilizing glucose oxidase on CH-prGO-AuNRs composite, these CH-prGO-AuNRs/ITO bioelectrodes demonstrate high sensitivity of 3.2 µA/(mg/dL)/cm(2) and linear range of 25-200 mg/dL with an ability to detect as low as 14.5 mg/dL. Further, these CH-prGO-AuNRs/ITO based electrodes attest synergistiacally enhanced sensing properties when compared to simple graphene oxide based CH-GO/ITO electrode. This is evident from one order higher electron transfer rate constant (Ks) value in case of CH-prGO-AuNRs modified electrode (12.4×10(-2) cm/s), in contrast to CH-GO/ITO electrode (6×10(-3) cm/s). Additionally, very low Km value [15.4 mg/dL(0.85 mM)] ensures better binding affinity of enzyme to substrate which is desirable for good biosensor stability and resistance to environmental interferences. Hence, with better loading capacity, kinetics and stability, the proposed CH-prGO-AuNRs composite shows tremendous potential to detect several bio-analytes in the coming future.

High-performance and high-sensitivity applications of graphene transistors with self-assembled monolayers.

Charge impurities and polar molecules on the surface of dielectric substrates has long been a critical obstacle to using graphene for its niche applications that involve graphenes high mobility and high sensitivity nature. Self-assembled monolayers (SAMs) have been found to effectively reduce the impact of long-range scatterings induced by the external charges. Yet, demonstrations of scalable device applications using the SAMs technique remains missing due to the difficulties in the device fabrication arising from the strong surface tension of the modified dielectric environment. Here, we use patterned SAM arrays to build graphene electronic devices with transport channels confined on the modified areas. For high-mobility applications, both rigid and flexible radio-frequency graphene field-effect transistors (G-FETs) were demonstrated, with extrinsic cutoff frequency and maximum oscillation frequency enhanced by a factor of ~2 on SiO2/Si substrates. For high sensitivity applications, G-FETs were functionalized by monoclonal antibodies specific to cancer biomarker chondroitin sulfate proteoglycan 4, enabling its detection at a concentration of 0.01 fM, five orders of magnitude lower than that detectable by a conventional colorimetric assay. These devices can be very useful in the early diagnosis and monitoring of a malignant disease.

Colorimetric detection of cholesterol based on enzyme modified gold nanoparticles

We develop a simple colorimetric method for determination of free cholesterol in aqueous solution based on functionalized gold nanoparticles with cholesterol oxidase. Functionalized gold nanoparticles interact with free cholesterol to produce H2O2 in proportion to the level of cholesterol visually is being detected. The quenching in optical properties and agglomeration of functionalized gold nanoparticles play a key role in cholesterol sensing due to the electron accepting property of H2O2. While the lower ranges of cholesterol (lower detection limit i.e. 0.2mg/dL) can be effectively detected using fluorescence study, the absorption study attests evident visual color change which becomes effective for detection of higher ranges of cholesterol (lower detection limit i.e. 19mg/dL). The shades of red gradually change to blue/purple as the level of cholesterol detected (as evident at 100mg/dL) using unaided eye without the use of expensive instruments. The potential of the proposed method to be applied in the field is shown by the proposed cholesterol measuring color wheel.

QPRTase modified N-doped carbon quantum dots: A fluorescent bioprobe for selective detection of neurotoxin quinolinic acid in human serum

Highly fluorescent nitrogen doped carbon quantum dots (NCQDs) were synthesized using microwave assisted green method. It was characterized by Transmission Electron Microscopy (TEM), FTIR, UV-Visible absorption and Photoluminiscence (PL) techniques. The NCQDs were immobilized with an enzyme named quinolinate phoshphoribosyl transferase (QPRTase). The NCQDs immobilized by QPRTase was used as a fluorescent bioprobe for the selective detection of endogenous neurotoxin quinolinic acid (QA) whose elevated level in serum is marker of many neurological disorders such as Alzheimers, Huntingtons and HIV associated dementia (HAD) as well as deficiency of vitamin B6. Steady state PL studies were carried out to measure the PL response of the fabricated fluorescent bioprobe as a function of QA concentrations in human serum samples. This probe was found applicable in linear range [3.22-51µM] with the limit of detection ~ 6.51µM. It has desirable sensitivity ~ (0.02340±0.0001) µM-1, excellent stability for ~ 7 weeks and good reproducibility. The similar response of this fluorescent bioprobe for QA detection in triple distilled water and human serum shows that it is unaffected by variation in media. Hence, this fluorescent bioprobe can be employed for QA detection in serum sample for the early detection of many diseases.