Simpler, more cost-effective biosensing

Charles T. Troy, charlie.troy@photonics.com

There are two challenges facing optical label-free biosensing, especially for clinical applications, according to Alexandre Dmitriev of Chalmers University of Technology.

“The system should be able to work in physiological fluids – blood, urine, saliva – which are loaded with all sorts of proteins, creating strong, unwanted background for the biosensor signal,” he said. “And the instrumentation should be simple and compact enough to be placed at a point-of-care location and operated by medical – but not research – professionals.”

To improve optical label-free biosensing and make it more cost-effective, Dmitriev and colleagues at Chalmers have come up with a system that employs a vertical-cavity surface-emitting laser (VCSEL)/micro-optoelectromechanical system (MOEMS), along with a specially designed nanoplasmonic sensing chip and a CCD detector.

Schematics of the biosensing setup. The VCSEL/(MOEMS) micro-optoelectromechanical system light source, at bottom, illuminates the sensor surface (an array of gold nanoplasmonic disks on glass, enclosed in a simple fluidic chip). The CCD, at top, further detects transmitted light. Note how the diagram indicates the reduction of VCSEL light intensity after it passes the fluidic cell. Courtesy of the American Chemical Society.

The researchers designed a compact biochemosensing platform based on a VCSEL that generates coherent illumination at 850 nm. Rather than using a commercial one, the group developed its own. Fabricated in an in-house process, it is oxide-confined, with an oxide aperture diameter of 3 to 4 µm. During measurements, the VCSEL drive current was set to approximately 0.8 mA (Its threshold current is approximately 0.3 mA), and the beam divergence was 12° at full width half maximum, which was independent of the drive current.

A microlens, centered above the laser, was used to initially focus the outcoming light into a low divergent beam of approximately 600 µm in diameter as detected at the CCD chip of the regular monochrome Hamamatsu C3057 CCD camera, placed about 10 cm above the laser in the forward direction. A damping filter was inserted in front of the camera to prevent it from saturating.

“Among the developed biochemosensing strategies, those based on optically active nanostructured noble metals that support localized surface plasmon resonances in the visible and near-infrared spectral range are particularly attractive, as they provide the possibility of label-free detection of biological and chemical species with extremely high sensitivity,” Dmitriev said. “Additionally, such systems allow the possibility of easy integration.”

In explaining the reason for the research, Dmitriev said, “Our motivation was to address the second issue and to ‘strip’ optical label-free plasmonic biosensing of as many optical components as possible, leaving a still fully functional, highly sensitive analytical biosensing device. So in this work we avoided using any optical components – microscope set-ups, lenses, etc. – having a simple fluid handling cell, a single-wavelength light source and a CCD light detector. Note that all the mentioned components can be integrated in a compact vertical stack. From the biosensing perspective, there is no need for any extra distance between the light source (VCSEL), the fluidic cell with the plasmonic sensing chip and the surface of a CCD detector.” Dmitriev pointed out that these components are also cheap and widely available.

“Of course, the smart design of the plasmonic chip was required to maximize the performance,” he added. “Yet again, the chip itself was made with a very affordable technology: bottom-up large-scale nanofabrication (hole-mask colloidal lithography).

“Nonetheless, despite the simplicity, we straightforwardly achieved very sound sensitivity levels in the detection of protein-protein interactions.”

The group’s results were published in the Feb. 15, 2010, issue of Analytical Chemistry.