Near-infrared fluorescent (NIRF) materials are promising labeling reagents for sensitive determination and imaging of biological targets. In the near-infrared region biological samples have low background fluorescence signals, providing high signal to noise ratio. Meanwhile, near-infrared radiation can penetrate into sample matrices deeply due to low light scattering. Thus, in vivo and in vitro imaging of biological samples can be achieved by employing the NIRF probes. To take full advantage of NIRF materials in the biological and biomedical field, one of the key issues is to develop intense and biocompatible NIRF probes. In this review, a number of NIRF materials are discussed including traditional NIRF dye molecules, newly developed NIRF quantum dots and single-walled carbon nanotubes, as well as rare earth metal compounds. The use of some NIRF materials in various nanostructures is illustrated. The enhancement of NIRF using metal nanostructures is covered as well. The fluorescence mechanism and bioapplications of each type of the NIRF materials are discussed in details.

Cyano-substituted oligo (alpha-phenylenevinylene)-1,4-bis(R-cyano-4-diphenylaminostyryl)-2,5-diphenylbenzene (CNDPASDB) molecules are studied in solution and aggregate state by time-resolved fluorescence techniques. CNDPASDB exhibits a strong solvent polarity dependent characteristic of aggregation-induced emission (AIE). By time-dependent spectra, the gradual transition from local excited state to intramolecular charge transfer state with the increasing solvent polarity is clearly resolved. The transition time in high polarity solvent DMF is very fast, around 0.5 ps, resulting in a low fluorescence quantum yield. While in aggregate state, the intramolecular torsion is restricted and the local environment becomes less polar. Thus, the intramolecular charge transfer state is eliminated and efficient AIE occurs.

CdTe quantum dots (QDs) were synthesized using oleic acid (OA) and thioglycolic acid (HS-CH 2 COOH, TGA) as capping reagents. Biexciton Auger recombination of CdTe QDs was examined by femtosecond transient absorption spectroscopy by changing excitation intensity. The lifetime of biexciton Auger recombination τ Auger was analyzed as a function of QD diameter D and capping reagents. It was found that τ Auger is proportional to D R , and that the scaling index R is dependent on capping reagents and/or surface conditions. For TGA capped CdTe QDs, R is determined to be 4.6, whereas it is 7.0 for OA capped CdTe QDs with high luminescence quantum yields Φ of 60-85%. This relationship did not hold for small size OA capped CdTe QDs with rather low Φ of 15-30%, in which τ Auger became as short as 2∼3 ps irrespective of the diameter. These results suggest that biexciton Auger recombination of CdTe QDs is strongly dependent on the QD surface trapping and capping reagents.