Strategies for the Development of Metallic-Nanoparticle-Based Label-Free Biosensors and Their Biomedical Applications.

Label-free biosensors offer accurate sensing capabilities due to the reliable quantification of biological and biochemical processes. These devices function by establishing a dynamic interaction of analyte and receptor molecules and convert this interaction into a measurable signal through a transducer. In recent decades, label-free biosensors have attracted attention in biomedical applications due to the ease of linking nanomaterials with bioreceptor molecules. In this review, recent advances in sensitivity, specificity, and sensing mechanism related to label-free biosensors of metallic nanoparticles of gold, silver, aluminium, copper, and zinc oxide are presented. Selected sensing methods based on fluorescence, surface plasmon resonance, surface-enhanced Raman scattering, metal-enhanced fluorescence, and electrochemical sensors are discussed. New measurement techniques and rapid progress of label-free biosensors are going to play a vital role in the real-time detection of biomarkers in clinical samples, such as blood plasma, serum, and urine, as well as in targeted drug delivery. Future trends of these label-free biosensing mechanisms and their development are also discussed.

C60 Cheese Sticks: Supramolecular Architecture of an Amphiphilic Fullerene Pentapod.

An amphiphillic fullerene pentaphenol derivative, C60(4-HOC6H4)5H, in an aprotic solvent N-methyl-2-pyrrolidone (NMP), spontaneously self-assembles to a hitherto unknown cheese stick-like (CS) structure. This fascinating structure is observed only for C60(4-HOC6H4)5H and only in dry NMP solvent. The reason for such unique self-assembly behavior is ascribed to solvating power typical of NMP. The CS structure is observed in a narrow concentration range of the solution. This indicates that there exists between the solute and the solvent an optimal interaction condition for CS to remain stable.

Dual function of a living polymerization initiator through the formation of a chain-end-protecting cluster: density functional theory calculation.

Sodium benzanilide (Na(+)BA(-)) initiators have opened a new route to living anionic polymerization of n-hexylisocyanate (HIC) with 100% yield and controlled molecular weight. The NaBA initiators not only provide initiation points for polymerization by attacking HIC monomers but also successfully prevent back-biting side reactions without any help from additives. Our hypothesis on this dual function of the NaBA initiators is that they self-assemble to form protection shields around the chain ends. Indeed, our density functional theory calculations performed under experimental conditions on the free energy of formation of (NaBA)n clusters of various sizes and conformations searched by Monte Carlo simulations show that the BA(-) moiety forms a stable complex with Na(+) in a fan-like circular-sector shape owing to its double binding sites (N(-)-C=O ↔ N=C-O(-)) and that the tightly-bound NaBA units spontaneously self-assemble to form small (NaBA)n clusters (n = 2 and 4). The growing end of the polymer chain [(BA)(HIC)n(-)], which resembles BA(-), would also assemble with n - 1 NaBA units to form an n-mer cluster. We expect that the chain end in this cluster would be more available to attack small HIC monomers coming into the cluster (leading to chain growth) rather than folding back to attack the middle of the chain (leading to cyclotrimerization to isocyanurates and depolymerization).

Molecular level ordering in poly(2-vinylpyridine).

The reaction between atactic poly(2-vinylpyridine) and 1,4-dibromobutane leads to formation of long-range 3D molecular ordering in polymer chains mainly because the side group (pyridine) of the polymer chain changes to a syndotactic configuration. This may enable the production of functional molecular devices that operate on a 3D atomic scale.

Solvent-induced transition of hollow sphere to giant-tube from amphiphilic rod-coil-rod triblock copolymers of 2-vinylpyridine and n-hexyl isocyanate.

The effect of solvent compositions on the micellization behaviors of amphiphilic poly(n-hexyl isocyanate)-b-poly(2-vinylpyridine)-b-poly(n-hexyl isocyanate) (PHIC-b-P2VP-b-PHIC) rod-coil-rod triblock copolymer was studied. In absolute methanol the block copolymer formed hollow spherical micelles. These micelles transformed into giant-tubes by simply changing the solvent composition. With 20% THF in CH3OH hollow spheres got interconnected and formed species that are precursors of the giant tubes. When the composition of THF reached 50-70%, long giant tubes are formed. Further increasing THF content in the mixed solvent, fragmentation of the wall of the tubes was observed. In absolute THF, which is the common solvent for both the blocks, the usual phase separation occurred. TEM image of the giant tubes stained with iodine showed that the core of the tubes is made from PHIC rod block.

Determination of the size of water-soluble nanoparticles and quantum dots by field-flow fractionation.

Field flow fractionation (FFF) technique is used to determine the size of water-soluble Au, ZnS, ZnS-Mn2+ nanoparticles, and CdSe, CdSe-DNA quantum dots (QDs). The results of the FFF measurements are compared with the particle size analysis using conventional techniques like scanning electron microscopy (SEM), transmission electron microscopy (TEM), and dynamic light scattering (DLS) studies. Water-soluble gold nanoparticles (AuNPs) stabilized by mercaptosuccinic acid (MSA) as the ligand when analyzed by the SEM and DLS showed evidence of extensive aggregation, preventing an accurate determination of the average particle size. The TEM analyses without staining offered a facile measurement of the nanoparticle core but average particle size determination required analysis of the TEM image using image analysis software. On the other hand the FFF is seemingly a convenient and easy method for the determination of the average particle size of the AuNPs. In case of the ZnS and ZnS-Mn2+ nanoparticles with mercaptopropionic acid (MPA) as the capping agent severe aggregation prevented accurate estimation of particle sizes even by the high resolution TEM (HRTEM), where as the size determination by the FFF was very facile. Analysis of the CdSe-DNA conjugate by the TEM was difficult as the sample got damaged upon exposure to the electron beam. The FFF cross-flow condition is apparently noninvasive and hence the technique was very effective in characterizing the CdSe-DNA QDs. Furthermore, using this simple technique it was possible to fractionate a sample of the AuNPs. The FFF measurement of water-soluble nanoparticles is an excellent complement to characterization of such particles by the conventional tools.

Characterization of host-guest complexes of cucurbit[n]uril (n = 6, 7) by electrospray ionization mass spectrometry.

When an intramolecular cavity exists in a molecule, it can trap another chemical species to form a host-guest complex. We examine the formation of such an inclusion complex with cucurbit[n]uril (CBn, n = 6, 7) as the host to trap alkali metal or ammonium ions as the guest, by electrospray ionization mass spectrometry (ESI-MS). The results show that the inclusion complexes are formed between the three-dimensional cylinder of CBn hosts and the guest cations. Selectivity of the complex formation is dependent both on (1) ion-dipole interactions between the cylindrical portal of the CBn hosts and the guest cations and (2) the hydrophobic interactions at the inner cavity of CBn.

Unprecedented control over polymerization of n-hexyl isocyanate using an anionic initiator having synchronized function of chain-end protection.

The living anionic polymerization of isocyanates carried out using conventional initiators is associated with side reactions due to rapid initiation rates as well as back-biting by the growing chain, resulting in a lack of control on the molecular weight (MW) and molecular weight distribution (MWD) of the polymers. Successful control over the reaction was possible by using additives that could prevent back-biting. We find an initiator in sodium benzanilide (Na-BA), which has a slow initiation rate combined with additive function, so that use of an external additive is eliminated. The initiator has resulted in polymers with high yields and an unprecedented control over the MW and MWD. It is possible to introduce a number of functionalities at the termini of the polymer by using Na-BA derivatives as well as suitable terminating agents, leading to macromonomer, reactive and chiral polymers, and chiral macromonomer in approximately 100% yields. In the process, the finding has expanded the scope of polyisocyanates in diverse applications.