Special Report: Application-driven nanotechnology
The University of Puerto Rico at Rio Piedras hosts a vibrant multidisciplinary community of experts in nanoscale science and technology doing leading-edge materials research from energy to biomedical applications.
The emergence of antibiotic-resistant bacteria is a major threat to worldwide public health. Functionalised nanoparticles can offer novel solutions for the post-antibiotic era.
A) Ag-GQDs nanocomposite to combat drug-resistant bacteria
In this context, we developed a nanocomposite of silver nanoparticles decorated with graphene quantum dots (Ag GQD) and functionalised with polyethylene glycol. This nanocomposite inhibits the growth of Gram-positive S. aureus and gram-negative P. aeruginosa bacteria, which are commonly isolated from microbial infections in wounds. The concentration required to inhibit both types of bacterial strains is well below the concentration that would affect the mammalian cell viability, being therefore biocompatible. The effectiveness of the composite is due to a synergistic effect in which the GQDs facilitate the internalisation of the silver nanoparticles into the bacteria, thus reducing the overall concentration of silver required to produce the bactericide effect. These results suggest that Ag-GQDs can be used in the fabrication of antibacterial ointments, self sterile textiles, and sterile personal care products. Collaborations are sought to expand the current study, independently validate the results, and develop antibacterial products based on Ag-GQDs.
Contact: Dr Gerardo Morell, gerardo.morell@upr.edu,
+1 787 282 7047
B) 1D Fe3O4-based building blocks for MRI contrast agents
Superparamagnetic iron oxide particles (SPIO) are employed as contrast agents in magnetic resonance imaging (MRI) due to the fact that they exhibit clinically proven biocompatibility and high relaxivities. As T2 contrast agents, SPIOs alter the transverse relaxation times of water protons, providing dark negative signal intensity to images, and they enable the visualisation and monitoring of cells grafted in organs due to their high sensitivity and excellent clinical biocompatibility. In order to enhance the relaxivity, blood circulation half-life, and homing of MRI contrast agents to tumours, we developed 1D nanoscale core magnetite particles with high aspect ratio. These nanoparticulate contrast agents contain a large number of superparamagnetic molecules that induce large relaxivities (above 100mm-1s-1) and allow the visualisation of targets in the picomolar concentration range. Moreover, they can cross many biological barriers and interact with biological systems at the molecular level. We anticipate that the 1D Fe3O4 nanoparticles currently under development will enhance the MRI capability to image poorly vascularised tumours and even individual tumour cells, thus bridging a critical gap in this research field. Collaborations are sought to expand the current study, independently validate the results, and develop MRI products based on 1D Fe3O4 nanoparticles.
Contact: Dr Brad R Weiner, brad@hpcf.upr.edu,
+1 787 764 8369
C) Semiconductor ferroelectrics for photovoltaic devices
The photovoltaic (PV) effect observed in BiFeO3 (BFO) has received a great deal of attention due to its small band gap (2.7eV), large open circuit voltage (Voc), and switchable diode and photovoltaic effects. Diode-like rectifying characteristics can be easily obtained in single crystal BFO deposited on epitaxial substrates. The PV effects can be reversibly switched by applying an alternating electric field, and the sign of photocurrent related to the diode effect is opposite to the polarisation directions.
We synthesised and studied polycrystalline thin films of BFO co-doped with La and Ta or BLFTO. They show PV effects with a high oxygen vacancy concentration under an electric field much lower than the coercive field. We have shown that the electromigration of oxygen vacancies is responsible for such a phenomenon because it is hard to detect a switchable PV effect under such a low electric field in BLFTO thin films with a low oxygen vacancy concentration. We demonstrated that the sign of the photocurrent is independent of the direction of polarisation when the modulation of the energy band induced by oxygen vacancies is large enough to offset that induced by polarisation. We also found that the PV effect induced by the electromigration of oxygen vacancies is unstable due to the diffusion of oxygen vacancies and the recombination of oxygen vacancies with hopping electrons. Our results have provided deep insights into how oxygen vacancies and polarisation flipping affect the diode and PV effect in ferroelectric films. We are now expanding this investigation by performing bandgap engineering with suitable substitution of transition cations to cover a good range of the solar spectrum. Collaborations are sought for the fabrication multi-bandgap layered structures of BFO-based ferroelectric materials and study of their PV characteristics.
Contact: Dr Ram S Katiyar, rkatiyar@hpcf.upr.edu,
+1 787 751 4210
D) Unipolar resistive switching non-volatile memory
The resistive random access memory (ReRAM), with its simple design, excellent scalability, high speed storage capacity, low power consumption, and semiconductor process flow compatibility, has been identified by the International Technology Roadmap for Semiconductors as a potential memory technology. The memory effect in ReRAM is realised through reversible switching of the resistance of a material between two conductive states, high resistance state (HRS = off) and low resistance state (LRS = on), when an adequate electrical signal (voltage or current) is applied across it.
Initially, the application of a high voltage (known as forming voltage) switches the resistance of the device from HRS to LRS by the controlled breakdown at a limited current compliance. After the forming process is complete, the memory cell is switched back to HRS by applying a threshold voltage (Reset). Once in HRS, the memory cell can again be switched to LRS simply by a voltage sweep keeping the current compliance constant. Thus, the resistance of the memory cell can be switched repeatedly between the two states of low and high resistance provided the current compliance is correctly set. We demonstrated unipolar resistive switching suitable for non-volatile memory applications in polycrystalline BiFeO3 (BFO) thin films in planar electrode configuration with non-overlapping Set and Reset voltages and on/off resistance ratio of ~104, and good data retention (verified for up to 3,000s). The resistive switching mechanism in this planar device can be explained by two resistance states using the conduction filament (thermochemical) model. Collaborations are sought to move this technology forward and fabricate BFO-based prototype resistive switching memory devices.
Contact: Dr Ram S Katiyar, rkatiyar@hpcf.upr.edu,
+1 787 751 4210
Dr Gerardo Morell
Professor of Physics
University of Puerto Rico
tel: +1 787 282 7047
gerardo.morell@upr.edu
