Single- and multi-photon excited fluorescence from serotonin complexed with beta-cyclodextrin

Abstract

The fluorescence of serotonin on binding with beta-cyclodextrin has been studied using both steady state and time-resolved methods. Steady state fluorescence intensity of serotonin at 340 nm showed approximately 30% increase in intensity on binding with K(A) approximately 60 dm(3) mol(-1) and the fluorescence lifetimes showed a corresponding increase. In contrast, the characteristic green fluorescence ('hyperluminescence') of serotonin observed upon multiphoton near-infrared excitation with sub-picosecond pulses was resolved into two lifetime components assigned to free and bound serotonin. The results are of interest in relation to selective imaging and detection of serotonin using the unusual hyperluminescence emission and in respect to recent determinations of serotonin by capillary electrophoresis in the presence of cyclodextrin. The results also suggest that hyperluminescence occurs from multiphoton excitation of a single isolated serotonin molecule.

FIGURE 1

Fluorescence spectra from serotonin (5 μmol dm⁻³) excited at 300 nm in solutions at pH 7 containing:- 0 (curve 1); 5 (curve 2); 10 (curve 3) and 20 (curve 4) mmol dm⁻³ β-cyclodextrin.

FIGURE 2

Increase in fluorescence intensity from solutions of serotonin (5 μmol dm⁻³) at pH 7 on addition of β-cyclodextrin. The solid line indicates the fit with Ka= 53.2 dm³ mol⁻¹, as described in the text.

FIGURE 3

Nanosecond time-resolved fluorescence decays at 505 nm following multiphoton excitation at 750 nm of serotonin (1 mmol dm⁻³) in solutions (pH 7) containing β-cyclodextrin:- none (trace 1); 5 mmol dm⁻³ (trace 2); 10 mmol dm⁻³ (trace 3) and 20 mmol dm⁻³ (trace 4).

FIGURE 4

Fluorescence (hyperluminescence) lifetimes of the biexponential decay from serotonin (1 mmol dm⁻³) on multiphoton excitation at 750 nm and pH 7 with increasing β-cyclodextrin concentration. Emission was observed at 505 nm. Increasing the β-cyclodextrin concentration had no effect on the short lifetime component (■), whilst the lifetime of longer component increased (□). Also shown are the single exponential lifetimes of the UV fluorescence at 340 nm, excited at 300 nm (▲).

FIGURE 5

Intensity contributions in the biexponential decay of serotonin fluorescence (hyperluminescence) at 505 nm after multiphoton excitation at 750 nm with increasing β-cyclodextrin concentration. Relative intensity (percentage) contributions are shown for the short lifetime component (□) and the long lifetime component (■).