FLUORESCENCE MEASUREMENT

1 . A fluorescence sensor comprising:

a light source arranged for emitting light to a sample region, wherein the light source intensity is modulatable;

an indicator system located at the sample region, said indicator system comprising: a receptor for an analyte; and a fluorophore associated with said receptor, wherein the fluorophore has a fluorescence lifetime that changes in response to the presence of analyte at the receptor;

a single photon avalanche diode arranged to receive fluorescence light emitted from said sample region in response to the light incident on the sample region from the light source, and to generate an output signal;

a driver arranged to modulate the light source intensity at a first frequency;

a bias voltage source arranged to apply a bias voltage to the single photon avalanche diode, wherein the bias voltage is modulated at a second frequency, different from the first frequency, and wherein the bias voltage is above the breakdown voltage of the single photon avalanche diode; and

a signal processor arranged to determine information related to a fluorescence lifetime of the fluorophore based on at least the output signal of the single photon avalanche diode.

2 . A fluorescence sensor according to claim 1 , wherein the first and second frequencies differ by less than 10%.

3 . A fluorescence sensor according to claim 1 , wherein the signal processor operates on a component of the output signal of the single photon avalanche diode at a frequency given by the difference between the first and second frequencies.

4 . A fluorescence sensor according to claim 1 , wherein a signal generator is controlled to vary at least one of: the frequency difference between said first and second frequencies; and the phase difference between signals at said first and second frequencies used to modulate the light source and modulate the bias voltage.

5 . A glucose sensor comprising a fluorescence sensor according to claim 1 , wherein the analyte is glucose.

6 . A method of fluorescence sensing comprising:

emitting light to a sample region from a light source, wherein the sample region comprises an indicator system comprising: a receptor for an analyte; and a fluorophore associated with said receptor, wherein the fluorophore has a fluorescence lifetime that changes in response to the presence of analyte at the receptor;

receiving fluorescence light, emitted from said sample region in response to the light incident on the sample region from the light source, using a single photon avalanche diode, and generating an output signal;

modulating the light source intensity at a first frequency;

applying a bias voltage to the single photon avalanche diode, wherein the bias voltage is modulated at a second frequency, different from the first frequency, and wherein the bias voltage is above the breakdown voltage of the single photon avalanche diode; and

determining information related to a fluorescence lifetime of the fluorophore based on at least the output signal of the single photon avalanche diode.

7 . A method according to claim 6 , wherein the first and second frequencies differ by less than 10%.

8 . A method according to claim 6 , comprising determining the fluorescence lifetime information based on a component of the output signal of the single photon avalanche diode at a frequency given by the difference between the first and second frequencies.

9 . A method according to claim 6 , further comprising at least one of: varying the frequency difference between the first and second frequencies; and controlling the phase difference between signals at said first and second frequencies used to modulate the light source and modulate the bias voltage.

10 . A method according to claim 6 , wherein the analyte is glucose.

11 . A method according to claim 6 , wherein the fluorescence lifetime is less than 100 ns, preferably less than 30 ns.