Mid free detection is direct and relatively easy

Mid IR Integrated Photonic Devices for
biomedical Applications: Optical approaches are
widely used for chemical analysis in micro-systems as they have an excellent
track-record in chemical analysis, usually show the lowest limits of detection
(LOD) and provide the greatest chemical or morphological information of the
analyte.

There are a number of advantages in using optical
measurements such as:

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1) It is a non-destructive technique

2) More information can be collected with the same
sample

3) High sensitivity (ppb-ppm) and performance.

There are two most common
detection protocols used in optical bio-sensing are fluorescence based
detection and label-free detection. The target
molecules are labelled with fluorescence tags in the fluorescence based detection
techniques and the presence of the target molecule identified by the intensity
of the fluorescence. This technique provides extremely sensitive results but it
needs laborious labelling process. Its quantitative analysis is
challenging. On the other hand, Label free detection
is direct and relatively easy to perform. In this type
of detection technique contains refractive index (RI), absorption and
Raman spectroscopic detection. Raman detection
involves unlabelled target molecules like refractive index and absorption
detection, while the emitted Raman light is used for detection
which is similar to fluorescence based detection. Some devices based on real part of refractive indices
that lack specificity because lots of different parameter can change refractive
index. Both Raman and absorption infrared spectroscopies involves molecular
vibrational transitions, which is based on
scattering or absorption of molecules respectively. This
work is based on absorption spectroscopy because
the fundamental vibrations of biochemical molecules occur in this region, where
their absorption is orders of magnitude stronger than their overtone bands in
the near-infrared (NIR) making it useful for highly sensitive absorption
spectroscopy. A molecule can vibrate in many ways (symmetric and anti-symmetric
stretching, rocking, wagging, bending, twisting, scissoring) depending on the
number of atoms and its linearity. Each of these vibrations corresponds to a
vibrational mode. A diatomic molecule has one bond corresponding to one
vibrational mode. The symmetry of the molecule and a set of selection rules
define the allowed and forbidden vibrational transitions. For example, water (H2O)
is a non-linear molecule and has three vibrational modes and all of these are
allowed transitions or IR-active.

MIR is a part of the infrared region that has a
wavelength range from 2.5-20 ?m that corresponds to wavenumber range 4000-500
cm-1 and frequency range 120-15 THz. MIR is an important region for a
broad range of applications in various fields such as spectroscopy,
pharmaceuticals, forensic, security and safeguard, astronomy, climate and
pollution monitoring, sensing and detection of some prominent gases such as CO2,
O2, CO, NO, CH4, HCl, liquids such as oil, biological
tissues and solids such as plastics, food etc. These applications are not only
technologically significant but also contribute to a safe and secure society.

The main driving force behind this work is MIR
aqueous bio-medical sensing. MIR technologies are promising for identification
and detection of analytes of very low concentration using their fingerprint
absorption in this region, offering high selectivity over a wide range of
compounds without labelling. The presence of biological markers of disease or
of molecular changes in clinical samples can be distinguished by studying their
MIR spectra, leading to the rapid high sensitivity, real time measurement of
complex mixtures to identify and quantify species for point of care diagnosis.
However, realisation of optical biosensors by integrating a variety of active
and passive optical components into a monolithic planar structure will enable
improved efficiency and a compact device.

Integrated optics
allows different optoelectronic and microfluidic components to be fabricated on
a single chip to help in realizing sensing of liquid analytes. This feasibility
of monolithic integration makes the device robust and mass producible thus
making it a potentially low cost device. Vision of an integrated lab on a chip
sensor system comprised of a MIR light source, waveguides, a detector, a
microfluidic channel and an integrated Fourier transform infrared (FTIR)
assembly monolithically fabricated on a single substrate. The most common detectors
used for detecting MIR signal are mercury cadmium telluride (MCT) detectors
because of their broad emission spectrum covering the wavelengths region from 2
to 16 ?m, better sensitivity as compared to other common detectors used in FTIR
such as DLaTGS detector and better signal to noise ratio. There is a great
potential for integrating MCT detectors on the same chip along with the source
and waveguide as MCT planar waveguides have been demonstrated on CdTe
substrates.