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CMOS image sensor platform for time-resolved fluorescence measurements with europium

Quantitative readout of test strips demonstrates broad applicability in in-vitro diagnostics

IMMS presents the details and the operating principle of a chip platform developed at the institute in the video “CMOS image sensor platform for time-resolved fluorescence measurements with europium”. The aim is to use it to open up further applications in in-vitro diagnostics in future research and development projects beyond the exemplarily demonstrated quantitative reading of test strips.

Example application quantitative readout of test strips

In der In-Vitro-Diagnostik werden Zielanalyten zunehmend mit Fluoreszenzfarbstoffen markiert, da sie sich leicht von Hintergrund- und Störsignalen unterscheiden lassen. Das IMMS hat einen Lock-In-Imager-Chip für die zeitaufgelöste Fluoreszenzbildgebung mit Europium entwickelt und in eine Beispielapplikation zum digitalen Auslesen von Streifentests integriert. Diese auch Lateral-Flow- Assays (LFA) genannten Tests spielen eine wichtige Rolle für die In-vitro-Diagnostik. Sie sind kostengünstig, einfach zu handhaben und daher prädestiniert für die dezentrale und zeitkritische Diagnostik. Sie sind u.a. als Schwangerschafts- oder COVID-19-Schnelltests weit verbreitet, um qualitative Aussagen (positiv oder negativ) treffen zu können. Für viele diagnostische Fragen werden jedoch quantitative Aussagen zu Konzentrationen und Verhältnissen benötigt. Gängige LFA-Reader-Kombinationen mit klassischen Farbstoffpartikeln wie Gold sind dafür nicht empfindlich genug. Neue LFA-Reader-Kombinationen mit Europium-Markern bieten weitaus höhere Ausleseempfindlichkeiten, die durch die CMOS-Imager-Plattform unterstützt werden. Durch deren Lock-In-Prinzip können aufwändige optische Filter eingespart werden.

Operating principle of the CMOS image sensor platform for time-resolved fluorescence measurement with europium

Classic fluorescence measurement

The core of the platform is a five-transistor lock-in pixel, which IMMS has optimised for fluorescent substances that have a particularly long afterglow, such as the widely used europium-based dyes.

Classical fluorescence detection works with optical filters. Here, a UV light source illuminates an analyte that is marked with a fluorescent dye. In this case, this is europium which is excited by the UV radiation and produces a red fluorescent light. An optical filter then separates this light from the UV light and allows only the fluorescent light to reach the detector.

Time-resolved fluorescence detection

“Time-resolved fluorescence detection with lock-in pixels, as we have implemented it, does not require optical filters” explains Eric Schäfer, Head of Microelectronics at IMMS.“ Distinction between excitation and fluorescence light based on their colours is no longer possible - and we don't need it, because we use the different decay times of the light source and the fluorescent dye after switching off the light source.” While the excitation light decays within a few nanoseconds, the europium continues to glow for several 100 microseconds.

Lock-in-pixel principle

During excitation the pixel is deactivated, i.e. the charge carriers are dissipated. Afterwards, the pixel is activated and the charge carriers generated by the fluorescent light are collected in the pixel. Since the fluorescent light can be very little, it is useful to repeat this process and operate the pixel in the so-called lock-in mode. For this, the light source is pulsed and synchronously the pixel is always activated when the light fades away. This means that the fluorescence signal is being collected over several cycles and thus amplified. At the same time, the noise is reduced.

“We can adjust the number of cycles to the different light conditions: If we have a lot of light, a lot of fluorescent light, because we have a lot of analytes, for example, we can run fewer cycles. If we have relatively little, we can run more cycles” says Eric Schäfer. “This results in a very high dynamic range.” After the analyte has been excited several times and enough charge carriers have been collected, the sensor element is being read out and reset for the next measurement.

Imager platform for a wide range of applications in in-vitro diagnostics

“Since we have developed the sensor element as a pixel, we can use it to build entire image sensors, for example for optical imaging systems or for contact imaging” Eric Schäfer summarises.

The imager platform thus provides the basis for opening up a wide range of applications in in-vitro diagnostics. IMMS aims to use the platform to develop further application-specific time-resolved image sensors and the associated hardware and software modules.

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Dipl.-Hdl. Dipl.-Des. Beate Hövelmans

Head of Corporate Communications

beate.hoevelmans(at) (0) 3677 874 93 13

Beate Hövelmans is responsible for the text and image editorial work on this website, for the social media presence of IMMS on LinkedIn and YouTube, the annual reports, for press and media relations with regional and specialist media and other communication formats. She provides texts, photographs and video material for your reporting on IMMS, arranges contacts for interviews and is the contact person for events.