Measurement of Luminescence Decays: Methods and Instrumentation

by Dr. Mark Sulkes

Department of Chemistry, Tulane University, New Orleans, Louisiana 70118, USA.

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Developments in electronics, PMT technology, and excitation light sources have contributed to improvements in luminescence decay detection. The result in recent years has been more precise determination of lifetimes, particularly in the sub-ns regime, at often drastically reduced costs. The most important factor in cost reduction, also affording some enhanced capabilities, has been the development of numerous LED and laser diodes, spanning a wide wavelength range, that can provide sub-ns pulses and can be driven even to GHz frequencies. The cost of these light sources is typically a small fraction of conventional laser systems. The result in the lowest cost regime (waveform digitizer combined with LED/laser diode excitation sources) is a rather good and potentially quite inexpensive system for determining strong emission luminescence lifetimes from multi-ns to µs. This review is particularly concerned with time and frequency domain methods that are capable of more precise lifetimes in the ns to the sub-ns regime. These capabilities have been available for several decades at fairly significant costs. The advent of LED/laser diode excitation sources has greatly lowered the cost of these capabilities and even afforded some performance enhancements, particularly in the frequency domain. Powerful commercial instrumentation is available at much-reduced cost. User implemented or improved, systems at a further reduced cost are increasingly feasible.

1 Introduction

The measurement of luminescence decays as a function of time provides fundamental information for an enormous range of applications and systems in chemistry and biology.[1-3] Continuing technical developments have made possible precise luminescence decay measurements, even on sub-ns timescales, at increasingly lower cost. Partly, this is due to the availability of faster electronics at a decreasing cost. However, the single most important factor has been the development of LEDs and laser diodes that can be driven with narrow pulses in time and at high frequencies - often replacing expensive laser systems. The result has been powerful commercial systems at lower cost. There are also growing opportunities for surplus, homemade, and home modified components with excellent performance at a far lower cost.[4] The other limiting factor is detector - photomultiplier (PMT) - performance. Ongoing PMT developments have brought about improved performance at lower prices. In addition, past technical publications have shown how to get greatly enhanced performance from common and inexpensive models.

This review of instrumentation developments will mainly consider the time domain method of time correlated single photon counting (TCSPC)[1,2] and the frequency domain method of phase modulation fluorometry[1,2]; some other methods and equipment will be mentioned more briefly.

2 History

Apparently, the first luminescence lifetime measurements were reported in 1926, using phase fluorometry—ns timescale decays obtained by measuring phase delays (albeit lifetime values with somewhat limited precision).[5] Continuing early work employed phase modulation fluorometry. Lakowicz recounts some early developments [1]; Klein records various experimental benchmarks in the method from the 1920s to 1980s.[6]

Time domain measurements advanced with the development of instrumentation and methods for recording luminescence as a function of time. Here are some benchmarks:

• first photomultiplier tubes (the 1930s)[7]

• first oscilloscopes (the late 1940s)[8]

• Polaroid cameras (the late 1940s) → oscilloscope photographs of decays

• time gated boxcars for recording transients (beginning late 1950s to 1960s)[9]

• sampling/signal averaging methods (beginning in early1960s- the early 1970s)[10]

• first TCSPC (1971)[11]

• µs timescale digitizer recording of decay waveforms (beginning in the late 1970s)

• ns timescale: optical recording of waveforms (Tektronix 7912, the late 1970s)[12]

• ns timescale digitizer recording of decay waveforms (beginning in the mid-1980s)

• streak cameras for ns and ps decays (beginning in the early 1980s)[13]

• laser based ultrafast upconversion detection, to fs (beginning in late 1980s) [1,14,15]

At present, commercial instrumentation is available for both time domain and frequency domain luminescence decay measurements. Although they have different operating principles, both kinds of instruments have similar performance quality criteria for light excitation sources and light detection.

3 Time Domain Methods

3.1 Methods other than TCSPC

Following excitation, luminescence intensity is recorded as a function of time. All of the methods in the benchmark list except for streak cameras normally employ PMT detectors. For the recording and averaging of analog signals, triggered waveform digitizers have essentially supplanted other methods. Given the current relatively low cost of a >300 MHz digitizer as new equipment and the extremely low cost in the used equipment market, the advent of pulsed LED/laser diode excitation sources makes this is a rather low cost method for obtaining an overall system response of several ns or less;[4] PMT transit time spread (TTS) generally is the limiting factor.

Streak cameras offer excellent capabilities but at high prices. Following the production of photoelectrons at a cathode, a streak camera then deflects the photoelections via a transverse electric field that increases in strength with time; the transversely deflected photoelectrons are amplified by microchannel plates before impacting on a phosphor.[13,16] The 2D images are then optically recorded. The use of CCD cameras allows for detection at a photon counting limit.1[16] Such a system is very powerful: It allows for acquisition of 2D (wavelength, time) data, with high sensitivity, down to ps. Ultimate resolution would be obtained with light excitation widths of a few ps to sub-ps, requiring expensive laser systems. Sub-ps resolution has been reported with a photon counting streak camera.[17] The limiting factor, aside from the cost of any such laser sources, is the cost of the detection system - well over 10[math]^{5}[/math] $US.

Laser based fluorescence upconversion detection has a time resolution limited by the laser pulses, potentially down to fs.[1,14,15] However, these methods are quite expensive and require specialized expertise.