Biosensing microsystems: fast, label-free, real-time clinical testing
Most contemporary clinical diagnostic tests are based on time-consuming, expensive, and sophisticated techniques performed by specialized technicians in laboratory environments. Furthermore, these techniques typically require labeling of the samples with fluorescent or radioactive markers. Given the complexity of these procedures, there is an urgent need for methods that can provide fast, simple, and cost-effective analytical testing. Advances in micro- and nanobiosensors offer the possibility of diagnostic tools with increased sensitivity, specificity, and reliability for in vivo and in vitro analytical applications. These sensors also afford the possibility of taking different measurements in parallel and integrating several analytical steps, from sample preparation to detection, into a single miniaturized device (i.e., lab-on-a-chip).
Our devices are based on integrating optical label-free biosensor arrays within microfluidic networks. The biosensors themselves rely on evanescent field principles to achieve high levels of sensitivity. When an interaction between a receptor molecule—previously immobilized on the core of the sensor (i.e., waveguide surface)—and its complementary analyte occurs, this produces a change in the optical properties of the propagated light within the waveguide via the evanescent field (see Figure 1). The highest surface sensitivity is obtained in waveguides with a high contrast refractive index between the core and the substrate as well as a small-dimension core thickness.
To detect changes in the optical properties of the propagated light produced by the biosensors, we have chosen a highly sensitive Mach–Zehnder interferometer (MZI) device1 constructed from rib waveguides of nanometer dimensions: see Figure 2(b). In this configuration, the guided light is split into two arms, and after a certain distance they are recombined again in an output optical waveguide. In one of the arms, a sensor area houses the specific bioreceptor. The phase shift between the two arms is measured to determine the extent of the molecular interactions occurring in the sensor area. To obtain single-mode behavior, the parameters of the waveguide must be carefully chosen.
The sensor has been applied to real-time and label-free detection of single DNA point mutations in the BRCA-1 gene, which can identify women who are predisposed to developing breast cancer. The MZI-based sensor successfully discriminated between normal and mutant sequences and demonstrated a limit of detection in the picomolar range for fully hybridized DNA and a limit of detection in the nanomolar range for single-point mutations.
For the lab-on-a-chip development, the sensors have been integrated with a microfluidic network at the wafer level. The microflow cells—see Figure 2(a)—consist of a three-dimensional embedded microchannel network fabricated using enhanced CMOS-compatible SU-8 photoresist multilevel polymer technology on top of a wafer containing the MZI nanophotonic biosensor devices. The platform can operate at pressure drops up to 1000kPa under steady-state flow rates ranging from 1 to 1000μl/min in the laminar flow regime.2
Further development of biosensing microsystem platforms based on the integration of MZI, microfluidics, and optical detectors will lead to significant advances in clinical diagnostics. Currently, we are developing the technology for direct fabrication of grating couplers onto the waveguides for light diffraction and coupling, which promises significant increases in sensitivity.3 In the future, fabrication of nano/macrosystems with hybrid integration of light sources, photodetectors, transducers, microfluidics, and electronic functions will offer the possibility of diagnostic tools with increased sensitivity, specificity, and reliability for early disease detection.
Laura M. Lechuga is the head of the Nanobiosensors and Molecular Nanobiophysics Group at CIN2. Her group has pioneered developments in integrated interferometer biosensors, magnetoplasmonic sensors, and optical microcantilever devices.
Orlando Hidalgo is a researcher in the Nanobiosensors and Molecular Nanobiophysics Group at CIN2. He was previously a professor in the General Physics Department at Havana University.
