LEDs for Fluorescence

by James Beacher, CoolLED Courtesy of Michael W. Davidson, Florida State University.

LEDs offer reliability and repeatability for fluorescence applications.

ntil recently, excita- tion has been achieved using U broad- gas dis- charge lamps with a combination of op- tical filters to remove many of the un- wanted . These lamps are widely used and accepted in microscopy applications. The devices’ wide bands of excitation (from the UV across the visi- ble spectrum) render them useful for most fluorescence applications. These arc lamps use a combination of high temperatures and pressure, meaning that, although the occurrence is rare, mercury bulbs can ex- plode. Metal halide lamps combine mer- Figure 1. The operating intensities and lifetimes of various sources are shown. cury with other metals to produce their light, and they address some of the dis- advantages of gas discharge lamps, but they are not the ultimate . All gas discharge lamps take time to reach an operating equilibrium. Once this is achieved, the lamp requires a period of waiting after it is turned off before the bulb can be reactivated. As a result, lamps often are kept running all day to permit immediate use when required. This con- tributes heat to the area, which can cause imaging problems. Lamp intensity fluctuates during use and decays throughout its lifetime, which can be as short as a few hundred hours (Figure 1). As a result, both qualitative and quanti- tative data can be unreliable. As pressure is increasing to produce repeatable results and to satisfy the FDA’s good-practice guidelines (GxP), gas discharge lamps may introduce uncertainty into otherwise excellent results. Figure 2. This bare LED die emits high intensity in all directions. However, recent advances driven by

Reprinted from the February 2008 issue of Biophotonics International © Laurin Publishing Co. Inc. A Laurin Publication

Photonic for Biotechnology and LEDs

mass market applications for LED tech- nology, such as domestic and automo- tive , are making it possible for LEDs to replace lamps as an excitation source for fluorescence microscopy. Thus, biologists can benefit from the advan- tages of these light sources.

LED basics An LED chip (die) is a semiconducting material doped with impurities to create a p-n light-emitting junction (Figure 2). Typically, the die are encapsulated to form a simple optical package. When an elec- trical current is applied to the LED, the of the released at the junction is determined (inter alia) by the materials used. Thus, LEDs produce wavelengths of a defined band- width around 10 to 30 nm. The light intensity of LEDs varies over the visible spectrum, with peaks in the vi- olet, and red regions being particu- larly strong. Because there have been few mass market applications requiring high- power peaks in the , yellow and or- Figure 3. These mouse embryonic fibroblasts were stained with Al 568 antibody (red) and ange regions, these spectral bands have centrine GFP (green). The DNA was stained with DAPI (blue). The images were taken with the experienced less development to date. PrecisExcite using a 100-ms exposure. The 400-nm excitation was set at 10 percent, the However, as the number of applications 465-nm at 5 percent and the 525-nm at 80 percent. Courtesy of the Institute of Molecular for LEDs has grown, intensities in this re- Pathology in Vienna, Austria. gion have increased also. LEDs have been used in biological re- search for fluorescence excitation since the early 1990s. Early attempts were lim- ited by their low efficiency and by the availability and suitability of commer- cially available products. Although LEDs have become more powerful, today’s brightest LEDs are still only 25 to 35 per- cent efficient, and attention must be given to thermal management to ensure their stability, reliability and lifetime. As a solid-state electronic device, an LED light source is capable of a precise level of control. No mechanical shutters or neutral density filters are needed to reduce the in- tensity of light falling on the sample when using LEDs because they can be instantly switched off and on and because light in- tensity is controllable in discrete (typically 1 percent) steps from 0 to 100 percent. This also means that vibration and re- sponse delay are reduced with the elimi- nation of these moving parts. The stability of LED intensity over short periods and throughout the device’s life- time is a key benefit in ensuring consistent Figure 4. Each LED array module contains up to 96 individual LED chips. LEDs and repeatable excitation. Although cur- trolled by a remote pod that sits beside This saves the cost of replacement bulbs rent LEDs are considered efficient, about the microscope to allow the user to switch and the time it takes for the difficult and 70 percent of their output is heat. To en- the wavelength and to adjust the inten- time-consuming realignment of gas dis- sure optimal stability, cooling them is es- sity while looking at the samples. How- charge lamps. sential. Active cooling while the units are ever, integration of the device within a on will hold them at an optimal temper- microscope or imaging software package Less sample damage ature for maximum excitation and sta- is possible, too, enabling automated rou- Any light source can photobleach bio- bility, which makes repeatable results pos- tines and greater benefit from the fast- logical samples that are exposed to it long sible regardless of the age of the device. switching capability. enough, but broadband excitation, in- Wavelength switching can be achieved The lifetime of an LED is measured in cluding UV, can damage a sample in a very electronically, making submillisecond tens of thousands of hours. Because it is short period. Even with wideband block- switching possible. Further optimization available instantly, the instrument can be ing filters, damage is not uncommon. is likely in the future, but switching times turned on for only the time needed to ex- Using the lower out-of-band transmis- of a few hundred microseconds already cite the fluorophore. We believe that, if sion — particularly low UV — available are available. an LED source is used in this way, its with LEDs reduces phototoxicity, per- LED light sources typically are con- effective lifetime could be indefinite. mitting users to image more often with- out damaging cells, which makes it easier to keep track of cell movement. It is ex- pected that, by using LEDs, one can achieve the same quality of imaging with less than 25 percent of the exposure time of shuttered lamps. Reduced autofluo- rescence from less exposure also produces crisper images. Light sources mounted directly to the microscope can introduce heat, vibration and electromagnetic interference, all of which can damage the samples and com- promise the results. In an LED system, electromagnetic interference can be re- moved from the sample and microscope with lightguides. This is attractive for elec- trophysiology and for work carried out in a Faraday cage. The inherent stability of thermally con- trolled LEDs is important for experiments that may run for many days or weeks. Rather than requiring lamp stability and lifetime to be factored in, LEDs make it possible for every image to be taken under identical excitation conditions from the beginning of the experiment to the end. Such conditions are particularly impor- tant in systems using automation and ro- botics, and LEDs offer the additional ben- efit of not needing to be replaced or realigned, as do bulbs. Dual procedures using fluo- rophores such as GFP with red fluores- cent such as mCherry are be- coming common. Here, controlling the intensity of individual wavelengths in- dependently allows excitation to be op- timized in a way that does not saturate GFP and that still excites the red fluores- cent well (Figure 3). Mechanical neutral density filters, shutters and lamps cannot provide this level of control. In addition, Förster resonance energy trans- Figure 5. A traditional excitation setup (top) has more moving parts than an LED fer (FRET) can be carried out effectively illumination configuration (bottom). using LEDs for cyan or yellow fluorescent LEDs protein excitation. Fast switching and sta- rently, the interchangeable modules are pability remains attractive for general and bility are important for FRET. available at 400, 445, 465, 505, 525, 595 standard applications. However, many Applications requiring very short peri- and 635 nm, and additional wavelengths cell biologists already recognize that most ods of excitation lend themselves to LED are under development. of their work is performed with a small light sources; for example, in aligning a Patented technology developed by the number of specific . A flexi- sample before imaging with a -scan- company enables maximum light trans- ble LED system optimized for their par- ning confocal microscope. fer from the LED to the liquid lightguide ticular application provides all of the ben- It is important for the user of an LED and, thus, to the sample under excitation. efits without the limitation of a gas light source to match the optical filters in Light is recombined at the microscope discharge lamp’s mechanical system. the microscope to the selected peak wave- using a collimator that fits into the light As LEDs become more powerful, and as length of the LED excitation wavelength. port. This arrangement provides two ben- more wavelengths become available, LED Although LEDs offer narrowband excita- efits: (1) heat, electromagnetic interfer- sources will present the same benefits as tion, reducing the level of unwanted light, ence and vibration are kept remote from a gas discharge lamp while offering greater there are still “tails” of emission that must the microscope and samples, and (2) the control and lower running costs. A sig- be removed to reduce unwanted back- lightguides act as excellent diffusers, nificant change to illumination on a mi- ground light in the fluorophore’s emis- achieving an even homogeneous field of croscope will be the redundancy of mov- sion region. excitation at the microscope. ing parts such as filter wheels (Figure 5). The PrecisExcite LED excitation system This will reduce costs and should decrease from CoolLED is designed for fluores- Future of LEDs image capture times dramatically. At that cence microscopy applications. The com- Most high-end applications need the time, an all-around safer, highly control- pany’s experience with semiconductor unique benefits of stability, repeatability, lable LED product will exist that will en- LED die enabled it to use optimized LED control and reduced sample damage pro- hance the quality of fluorescence mi- parts rather than the standard ones avail- vided by LEDs. Specific applications, such croscopy. o able to produce a light flux at the LED in as live-cell imaging, require the fast switch- 2 excess of 70 W/cm . The user can choose ing and reduced photobleaching of an Meet the author from a range of wavelengths by selecting LED light source. James Beacher is the business development an LED array module containing up to Gas discharge lamps will continue to manager for CoolLED in Andover, UK; e-mail: 96 individual LED chips (Figure 4). Cur- play a role because their broadband ca- [email protected].

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