emission of fluorescent dyes are quenched by CNTs when fluorescent molecules directly adsorbed on CNT surfaces. In this study, we investigated the fluorescence quenching ability of singleand multi-walled carbon nanotubes (SWNTs and MWNTs, respectively) by measuring the fluorescence quenching ratio of fluorescence emission when fluorescein-labeled single-stranded DNA (Fluor-ssDNA) is reacted with the hybrids of 30-mers of thymine (T30) and SWNTs/MWNTs. As the basesequences of Fluor-ssDNA, we adopted dT30 and dA30 (Fluor-T30 and Fluor-A30, respectively). Fluorescence measurements revealed that the fluorescence quenching ratios of the mixture of Fluor-T30 and T30-SWNT/MWNT were 28 5 3.1% and 36 5 2.0% relative to free fluorescein at the same concentration, respectively. On the other hand, the those of Fluor-A30 with T30-SWNT/MWNT hybrids were 11 5 1.9% and 325 1.9%, respectively. One of the explanations of fluorescence quenching might be exchange reaction of Fluor-ssDNA and T30 molecules on SWNT surfaces. We conducted fluorescence measurements at various temperatures in order to estimate the thermodynamic parameters of the exchange reaction. The change of the enthalpy was positive value, indicating that the exchange reaction is endothermic process. These results suggest that SWNT hybrids have good resolution for the base-sequences of Fluor-ssDNA, while MWNT hybrids evenly and strongry could quench Fluor-ssDNA regardless of its base-sequences.

Foods contain a plethora of aromatic molecules-natural colors, synthetic dyes, flavors, vitamins, antioxidants, etc.-that are luminescent, exhibiting prompt fluorescence or delayed phosphorescence. Although food autofluorescence has been used to detect specific contaminants (e.g., aflatoxins) or to authenticate specific foods (olive oil), much of the potential of using the optical luminescence of intrinsic molecules for sensing properties of foods is unrealized. We summarize here work characterizing the photophysical properties of some edible, and potentially GRAS (generally-recognized-as-safe), chromophores and especially their sensitivity to, and thus potential for sensing, various physical-viscosity, mobility/rigidity-or chemical-polarity, pH-properties of food known to reflect or be indicative of food quality, stability, and safety. A thoroughgoing characterization of and robust protocols for interpretation of the luminescent signals from edible chromophores can expand the repertoire of analytical techniques available to monitor quality, and even safety, of the food supply at various stages of production, distribution and storage or even at point of sale.

Flavonols are naturally occurring antioxidants with complex photophysical properties. Their emission sensitivity to polarity and hydrogen bonding ability of the local environment suggests their potential as luminescent probes for water activity. Water activity, aw, is an important parameter for ensuring food safety and quality, as it is a determining factor for microbial growth and biochemical reactions. This study focused on 3-hydroxyflavone (3HF), a synthetic molecule that constitutes the backbone of naturally occuring flavonols. However, the photophysical properties and sensitivity to aw of other flavonoid glycosides were also explored. All flavonols were tested in binary solvent mixtures of different aw. 3HF is an excited-state intramolecular proton-transfer probe that exhibits dual fluorescent emission bands corresponding to its normal (λem = 405nm) and photo-induced tautomeric (λem = 525nm) form. The normalized intensity of the photo-induced tautomer was sensitive to aw of binary mixtures, with a marked decrease in intensity at aw > 0.8, likely due to progressive aggregation of 3HF molecules. Additional sensitivity to aw was observed in terms of the location of the emission bands. The difference in wavelength between the normal (N) and tautomer (T) forms decreased monotonically at aw > 0.4 due to a hypsochromic shift of the tautomer band. The relative position of the T∗ towards N∗ band as a function of aw was modelled using a log-logistic function. A critical aw value above which the sensitivity of 3HF significantly increased was estimated based on the model. Quercetin and additional flavonoid glycosides were also responsive to changes in aw; e.g., quercetin's fluorescence intensity decreased at aw > 0.6. Although additional validation of these probes in model food systems is required, the available data support the potential use of flavonols as probes of aw in foods and edible pharmceuticals.