Semiconductor nanocrystals or Quantum Dots (QDs) are small crystalline grains of a few hundred to a few thousand atoms with diameters less than 10 nanometers. QDs exhibit size-dependent properties, including size-dependent luminescence, and continuous excitation by any wavelength smaller than the emission wavelength. Their absorption onset and emission peak shift to smaller wavelengths with decreasing size. In addition, QDs have symmetric and narrow emission peaks and their emission spectra do not change with the excitation frequency. Due to their unique properties, QDs have attracted attention as a replacement for organic fluorescent dyes in biosensor applications. Traditional tagging of biomolecules that employs organic fluorophores has significant limitations when compared to tagging using QDs. Organic fluorophores tend to have narrow excitation window, and often exhibit broad emission spectra with red tailing. This makes it difficult to observe different fluorophores using a single excitation source. Furthermore, simultaneous quantitative evaluation of relative amounts of multiple analytes present in the same sample is difficult due to spectral overlap. QDs have high quantum yields, large extinction coefficients, long fluorescence lifetimes, high sensitivity and excellent photostability.

For biological sensing applications, it is important to obtain high quality QDs that exhibit high luminescence emission intensity (i.e. high quantum yield). In addition to the quality of individual QDs, their concentration in a liquid sample plays a critical role in determining the observed intensity of emitted light that reaches the detector. The light propagation in concentrated colloidal suspensions of particles with sizes comparable to the wavelength of the radiation has been a subject of research for several years due to the nonlinear interaction between radiation and particles. In the case of QDs, the size of the particles is much smaller than the wavelength of both the excitation and emitted radiation. Since QDs absorb as well as emit light the behavior is potentially more complex. The absorption of the excitation beam by the particles and the re-absorption of emitted light by slightly larger particles are dependent on the particle concentration along the path of the beam.

In this study we estimate the optimal concentration, and average inter-QD spacing, that maximizes the apparent emission intensity from a liquid sample. This is useful for designing biosensors based on QDs with the purpose of maximizing their detection sensitivity.

Zinc selenide quantum dots (ZnSe QDs) were synthesized using the hot coordinating solvent technique. In a typical synthesis experiment, aliquots were extracted at 10, 20 and 30 minutes of reaction time to produce QDs of increasing size as evident by the red shift in the wavelength of the peak PL intensity with reaction time. The sizes of the three synthesized samples were determined to be 2.7nm, 3.2nm and 3.5nm, respectively. After synthesis, the as-prepared QDs were dispersed in butanol and their emission spectra were measured at various dilutions. Three fluorometers were used to confirm that the observations were not instrument dependent.

The effects of particle size on the optimal QD concentration were studied using a series of ZnSe QD samples of varying average diameters diluted in butanol. Each sample exhibited a maximum PL intensity at a certain QD volume fraction that decreased with particle size. To correct for the variation of particle number density at various dilutions, the peak PL intensity values were divided by the number of QDs in the dispersion and plotted versus QD volume fraction. The normalized PL intensity exhibits a plateau at low QD concentrations, corresponding to the apparent emission intensity from a single QD.

The optimal particle concentration was used to estimate the minimum possible distance between neighboring QDs in dispersion, before attenuation of the PL intensity arises due to high particle concentration. This information is very useful for maximizing the sensitivity of biosensors utilizing QDs as luminescent beacons.

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