In the last decade the use of nanomaterials has been having a great impact in biosensing. In particular, the unique properties of noble metal nanoparticles have allowed for the development of new biosensing platforms with enhanced capabilities in the specific detection of bioanalytes. Noble metal nanoparticles show unique physicochemical properties (such as ease of functionalization via simple chemistry and high surface-to-volume ratios) that allied with their unique spectral and optical properties have prompted the development of a plethora of biosensing platforms. Additionally, they also provide an additional or enhanced layer of application for commonly used techniques, such as fluorescence, infrared and Raman spectroscopy. Herein we review the use of noble metal nanoparticles for biosensing strategies—from synthesis and functionalization to integration in molecular diagnostics platforms, with special focus on those that have made their way into the diagnostics laboratory.

Fast excited state proton transfer reactions at the surface of anionic sodium dodecyl sulfate (SDS) micelles have been investigated using the photoacid 4-methyl-7-hydroxyflavylium (HMF) chloride as probe. The acidbase kinetics of excited HMF are straightforward in water, with biexponential fluorescence decays reflecting ultrafast deprotonation of the excited acid (AH + )* (k d ) 1.5 × 10 11 s -1 or ca. 6 ps) and diffusion-controlled protonation of the excited base A* (k p ) 2.3 × 10 10 L mol -1 s -1 at 20°C). In aqueous micellar SDS solutions, the kinetics are much more complex; triple exponential fluorescence decays are observed at all pH values and temperatures examined. The longest decay time (τ 1 ) 760 ps at 22°C), observed only for (AH + )* and uncoupled from the acid-base equilibrium, is assigned to excitation of HMF in orientations incapable of prompt transfer of the proton to water, i.e., that must rotate to expose the acidic OH group to water (k rot ) 1.2 × 10 9 s -1 or ca. 800 ps at 22°C). The other two decay times, τ 3 and τ 2 , are due to emission from the species involved in the acid-base reaction at the micelle surface. Deprotonation of (AH + )* is slightly slower in SDS micelles (k d ) 3.4 × 10 10 s -1 or ca. 20 ps) than in water. Two processes are operative in the back protonation of A*: (i) pH-independent unimolecular reprotonation in the initially formed geminate compartmentalized pair (A*‚‚‚H 3 O + ) (k r ) 8.8 × 10 9 s -1 ) and (ii) pH-dependent bimolecular protonation of A* via entry of an aqueous phase proton into the micelle (k p ) 1.6 × 10 11 M -1 s -1 ). Dissociation of the geminate pair (k diss ) 1.6 × 10 9 s -1 ) forms A* at the micellar surface. The present study thus provides a rather detailed kinetic picture of the initial steps involved in an ultrafast excited state proton transfer process at the surface of a typical anionic micelle.

Malvidin-3,5-diglucoside (malvin), cyanidin-3,5-diglucoside (cyanin), and pelargonidin-3,5-diglucoside (pelargonin) are among the most representative anthocyanins because of their abundance in the most common red flowers and fruits. Anthocyanin color is directly affected by the pH-dependent chemistry of the red (acid) form of these compounds, while anthocyanin photostability is a function of the photophysics of the first excited singlet state. In the present work, we employ laser flash photolysis and picosecond time-correlated single-photon counting to determine the dynamics of the proton-transfer reactions of these three anthocyanins in the ground [deprotonation rate constants, k d = 1.3 × 106 s-1 (pelargonin), 1.8 × 106 s-1 (cyanin), and 3.8 × 106 s-1 (malvin)] and first excited singlet state [deprotonation rate constants, k d = 4.3 × 1010 s-1 (pelargonin), 4.0 × 1010 s-1 (cyanin), and 1.6 × 1011 s-1 (malvin)], respectively. The ground- and excited-state proton-transfer rate constants for anthocyanins and for photoacids of the naphthol type are found to correlate with an empirical parameter related to the ionic character of the dissociable OH bond. The present results show that the typically weak fluorescence of the flavylium cation form of anthocyanins is due primarily to competitive ultrafast, adiabatic proton transfer to water. This process is highly efficient as an energy-wasting mechanism, as would be required by an in vivo role such as protection of plant tissues from potentially deleterious excess radiant energy.