Multimode or dedicated? That’s the question, when it comes to microplate readers. Though multimode readers are at the top of many labs’ wish lists, dedicated readers still have their place.

If you flick through the various flyers and brochures advertising microplate readers, you will find the word versatility repeated like a mantra. Though only on the market since the late nineties, multimode microplate readers offering versatile functions seem to have taken the number one spot in the microplate reader market. According to a report by the Californiabased market researcher Phor Tech, multimode instruments comprised about one-third of the readers purchased between 1999 and 2002. It’s a good guess that by today, multimode readers will have gained a far larger market share.

Modern multimode readers are capable of tackling the very latest scanning and reading techniques. Besides classical absorbance, fluorescence or luminescence detections, these jacks-ofall-trade easily master more recently developed techniques, including fluorescence polarisation (FP), time resolved fluorescence (TRF), fluorescence resonance energy transfer (FRET) and FRET’s “bioluminescence brother”, BRET (bioluminescence resonance energy transfer). Some models offer their users more then ten different modes of operation.

Mr. Handyman readers

Multimode readers are usually equipped with a whole bunch of filters and detectors. BMG Labtech’s PHERAstar, for example, contains two pairs of photomultiplier tubes (PMTs) and an additional photodiode for signal detection. The PMTs are arranged around an application-specific optical module that is equipped with excitation and emission filters, a dichroic filter and a beam splitter. Up to five bar-coded optical modules can be inserted into the reader by hand and the machine automatically selects the correct one for the desired assay protocol. If you choose, say, fluorescence mode, the excitation light coming from a xenon flashlamp excites fluorophores to emit fluorescence. The fluorescence beam is then split by a beam splitter and detected by a pair of PMTs. In absorption mode, however, a photodiode detects the light that passes through the sample well, while PMT1 acts as a reference channel measuring the intensity of the starting beam. Similarly, PMT1 and 2 are used for fluorescence intensity, fluorescence polarisation and luminescence modes, while PMT3 and 4 take over time-resolved fluorescence.

Most readers can handle multiple formats, including 96, 384- and 1536-well plates. Some instruments also read plates with smaller (1, 6 and 24) or higher (3456) amounts of wells. Labs dedicated to high throughput applications or the HTS departments of drug discovery companies are usually crazy about instruments that can be integrated into roboting systems. To meet this need, many manufacturers sell automation-ready instruments equipped with plate stackers, plate handling robots, robot access modules and all the necessary software. Reading speed is another crucial factor for HTS-labs. A read-out of a traditional 96-well plate takes a modern state-of-the-art reader approximately 10 seconds. HTS readers such as Perkin-Elmer’s ViewLux and Evotec’s plate::vision can read 1536-well plates in 30 and 20 seconds, respectively.

Though multimode readers have taken the lead, dedicated microplate readers still have their place. Typically they are used by labs performing special assays like homogenous time resolved fluorescence (HTRF). HTRF is a time resolved, fluorescence resonance energy transfer (TR-FRET)-based technology that uses the principles of both TRF and FRET. In a HTRF experiment a donor- and an acceptor fluorophore are brought together by a biomolecular interaction. The donor is usually europium cryptate with a long-lived emission. The donor transfers its energy via FRET to the acceptor only when both molecules are in close proximity. The transferred energy is released by the acceptor molecule as specific fluorescence that can be detected by a HTFR-microplate reader. HTFR-assays are applied in drug screenings to mea sure, for example, protein-protein and ligand-protein interactions.

Dedicated to luminescence

Dedicated readers also make sense in those luminescence detection assays that are widely used in immunoassays, drug screening, food analysis, gene expression studies and cell-based assays like BRET. They may be the right choice for labs searching for an instrument with high sensitivity and low cross talk between microplate wells at moderate prices. Usually, dedicated luminescence readers are equipped with injector modules to enable the addition of reagents necessary in, amongst others, flash luminescence assays.

But irrespective of reader type, dedicated or multimode, there is almost no experiment that can’t be analysed with a microplate reader. Our microplate reader survey over the next few pages will guide you to the model that best suits your assays.

(First Published: Harald Zähringer, Lab Times 04/2006. We expressly disclaim liability for this information.)