Protein-mediated synthesis, pH-induced reversible agglomeration, toxicity and cellular interaction of silver nanoparticles.

Casein, a milk protein, is used to produce biotolerable and highly stable silver nanoparticles with a fair control over their size without using any additional reducing agent. These silver nanoparticles undergo reversible agglomeration to form protein-silver nanoparticle composite agglomerates as pH approaches to the isoelectric point of casein protein (pI=4.6). These agglomerates can then easily be re-dispersed in alkaline aqueous media with no obvious change in their optical properties. The nanoparticles can withstand high salt concentration (~0.5M), and can also be freeze-dried, stored as dry powder and then dispersed in aqueous media whenever required. More interestingly, by controlling the concentration of casein protein and pH, it was also possible to control the self-assembly of silver nanoparticles to produce fairly uniform spherical agglomerates. The nanoparticles and their agglomerates were thoroughly characterized using UV-visible and FTIR spectroscopy, TEM, SEM and DLS, etc. Cytotoxicity of the hybrid materials was examined using a Resazurin based cytotoxicity assay. After determining the LD(50) using NIH/3T3 fibroblast cells, the cellular interaction of these hybrid nanoparticles was studied to examine the behavior of casein-coated nanoparticles for their potential bio-applications.

How colloidal nanoparticles could facilitate multiplexed measurements of different analytes with analyte-sensitive organic fluorophores.

Multiplexed measurements of several analytes in parallel using analyte-sensitive organic fluorophores can be hampered by spectral overlap of the different fluorophores. The authors discuss how nanoparticles can help to overcome this problem. First, different organic fluorophores can be separated spatially by confining them to separate containers, each bearing a nanoparticle-based barcode. Second, by coupling different fluorophores to nanoparticles with different fluorescence lifetimes that serve as donors for excitation transfer, the effective fluorescence lifetime of the organic fluorophores as acceptors can be tuned by fluorescence resonance energy transfer (FRET). Thus, the fluorophores can be distinguished by their effective lifetimes. This is an example of how the modification of classical functional materials has already yielded improved and even new functionalities by the integration of nanoparticles with hybrid materials. We outline future opportunities in this area.

Evaluation of quantum dots applied as switchable layer in a light-controlled electrochemical sensor.

Gold electrodes with switchable conductance are created by coating the gold surface with different colloidal quantum dots. For the quantum dot immobilization, a dithiol compound was used. By polarizing the electrode and applying a light pointer, local photocurrents were generated. The performance of this setup was characterized for a variety of different nanoparticle materials regarding drift and signal-to-noise ratio. We varied the following parameters: quantum dot materials and immobilization protocol. The results indicate that the performance of the sensor strongly depends on how the quantum dots are bound to the gold electrode. The best results were obtained by inclusion of an additional polyelectrolyte film, which had been fabricated using layer-by-layer assembly.