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Advanced spectral instrumentation

Analytical technics in chemical and biochemical measurement continue to develop as supporting equipment and components, to apply advances for measurement and sensing. Optics, photonics, spectroscopy, imaging and microscopy have seen a significant growth and changes in the last two decades, to enable the development of new methods of analysis. There is an appetite to make sensors small and portable, yet be extremely selective, and measure very low concentrations. A core part of this is integration that incorporates the ever changing  micro-computing, mobile handsets, transducers, robotics and then communications, with for example IOT [Internet of Things] or Ethernet.

Simple dedicated display of information, which is significant to the end user, with the more complex part accessible, is important. Speed of analysis becomes very significant  in design methods, for example reacting to bio-hazard, medical condition, environmental incident. Sustainable remote operation of sensors and networking of sensors is showing greater significant to complex interpretations.  3D printing is relatively novel and being researched for construction of low cost disposable sensors. Nano material structures are enabling new methods of sensing. Analysis of multiple samples at speed is a continuing objective. This can be an array of medical samples in a tray, or, an array of micro fluidic flow capillaries, for laboratory sensing. It can also be multiple autonomous sensors located significant distances apart and network multiplexed using modern data communications.

Fibre optics can also collect analytical light from many measurement locations simultaneously, and be multiplexed into a single analytical platform. Switching these fibre optic channels can be done by optical multiplexing, which remains relatively  large in size. Applying multiplexing on a small chip may be determined in the future by new MOEMS [ Micro Optical Electro Mechanical System ] spatial light modulation using liquid crystal on silicon [LCOS ].

It is, however, very important to recognise the fundamentals and challenges of optical analytical measurement. Developing and designing the biotechnology to provide an easy to read fluorescent label is an example of  high emphasis “selective chemistry”, whereas many fluorescent techniques searching for low concentrations, require the best of optical detection or spectroscopic instrumentation.

Advanced spectral instrumentation

Here the emphasis may be more on the advanced spectral instrumentation; Spectral  interpretation by computer with the chemistry and preparation being very different. This “chemistry” can be complex and more suited to a laboratory preparation before analysis, or it can be without chemistry with the spectrometer making a complex analysis on unprepared samples.   

This sets a scale map of condensed chemistry and optical analysis at  one scale end and complex chemistry and analysis on the other scale end; and then all that “mix”  in the middle scale.

There is a considerable evolution of low complexity sensor chemistries and biochemistries to enable selective isolation of items of interest. The application scope is extensive, ranging from medical point of care equipment, hazard security monitoring, networked environmental  remote sensing, agricultural  and food production efficiency, and much more.

In bio-diagnostics we see the use of methods including  biomarkers, labels, immobilised state antibodies, to provide an “end point state “ which can be measured by optical, electrochemical, and thermal means. The “end point state” is often a colour, or fluorescence, which can be measured using relatively simple optical instrumentation.

The availability of LEDs, OLEDs, and laser diodes provide small solid state devices, with compact supply, replacing the more traditional bulky tungsten, mercury, deuterium, and arc source devices. High gain photodiodes, CMOS and CCD detection are all small. The latest micro optical coating developments allow easy and high performance application of waveband filtering even coating individual pixels on array detectors.

Array spectrometers are now very small, and can be used rapidly to determine one or more items of interest. The light input to these can use fibre optics as an option, which allows remote measurement into extreme locations, where electronics  are not acceptable. Compact spectrometers, which are easily combined with small and portable sensor devices and have sufficient  performance to allow a host of spectral complex analytical interpretations, in support of the primary Lab-on-the-Chip chemistry. In such cases multiple wavelength data can be retrieved for more than one analysis, or comparative referencing can be provided, simultaneously.

Many of these single-channel sampling applications can become multiplied using a small camera; such as we find in our Smartphones today. Immunofluorescence measurement is one of many examples, where a simple camera can image multiple end points and make comparisons. All of these devices allow compact base or chip mounting, with connection to a computer, and its specific software algorithm, dedicated display, and then onwards communication. Such systems can be autonomous receiving data remotely and providing remote inbound control if required. A more advanced imaging system uses Hyperspectral imaging, where some or all detector pixels can provide spectra as well. These however are still relatively large.

Lab-on-the-Chip

The chemistry / biochemistry to provide the “end point state” can now be contained as a small “Lab-on-the-Chip “, integrated with the “on chip” optical instrumentation. Microfluidic chips use small capillaries for the flow lines with only tens to hundreds of microns internal diameter.

Fibre optics have compatible dimensions making interfacing of capillary and fibre possible.

The liquid flow network is designed for dedicated tasks and allows introduction of the sample to be measured, reagents and other pro-active liquids, with timed and proportionate mixing. Capillaries can be coated internally with catalytic or bioactive material, such as immobilised state antibodies.

The preparation of such dedicated  Lab-on-the-Chip devices requires a full workstation of micro volume pumps, multiple chip holders and development facilities: however when completed these very small chips are compatible in size and readily mounted as one overall compact platform with the analytical optics and electronics.

While this concept of compactness represents one sector end of a scale, where the analytical optics and chemistry is relatively simple, there remains an enormous analytical challenge at the other scale end, with the need for state-of-the-art spectrometers, lasers, and photonic devices. Extracting extremely small analytical optical signal from noise or background often requires ultimate performance in the instrument design. Optical collection and throughput has to be optimised, it requires the best in cooled detectors, selection of light source or laser, and best spectrometer stray light and aperture to be compatible with sample interfacing. At the same time, very fast spectral acquisition can be very important. Examples include ultra- low level fluorescence, fast decay phosphorescence, Raman applied to low concentrations or nanotechnology, remote measurement by LIBS. More recent developments in sample interfacing have been helpful; for example, the use of surface enhancement techniques using silver/gold metal nanoparticles give enhancement to a Raman signal.

The ongoing evolution of micro-electronics, nanomaterials and photonics

The evolution and merging of new nanomaterials, micro electronics and photonics will certainly continue to catalyse photonic sensors for point of application, with the latest advanced computing and data communication underpinning devices. There still remains the case for advanced analytical optical instrumentation operating a sensitivities at the limit of today’s research, which so far cannot be simplified. But maybe new micro chemistry will shift the role of the instrumentation.

Written by John H. G. Knight,  Managing Technical Director,                 Knight Photonics Ltd, Leatherhead, United Kingdom

Labels: Micro-Fluidics,Spectral Opto-Electronics,compact sensor,spectral detection,lab-on-the-chip,OLED,LED

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