Sensitive optical biosensors for unlabeled targets: a review.

The realization of miniaturized optofluidic platforms offers potential for achieving more functional and more compact devices. Such integrated systems bring fluid and light together and exploit their microscale interaction for a large variety of applications. The high sensitivity of compact microphotonic devices can generate effective microfluidic sensors, with integration capabilities. By turning the technology around, the exploitation of fluid properties holds the promise of highly flexible, tunable or reconfigurable microphotonic devices. We overview some of the exciting developments so far.

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Integrated optofluidics: A new river of light

Refractive index (RI) sensors based on optical resonance techniques are receiving a high degree of attention because of the need to develop simple, low-cost, high-throughput detection technologies for a number of applications. While the sensing mechanism of most of the reported RI sensors is similar, the construction is quite different from technique to technique. It is desirable to have a uniform mechanism for comparing the various RI sensing techniques, but to date there exists a degree of variation as to how the sensing performance is quantified. Here we set forth a rigorous definition for the detection limit of resonant RI sensors that accounts for all parameters that affect the detection performance. Our work will enable a standard approach for quantifying and comparing the performance of optical resonance-based RI sensors. Additionally, it will lead to design strategies for performance improvement of RI sensors.

On the performance quantification of resonant refractive index sensors

Resonance-based wireless power delivery is an efficient technique to transfer power over a relatively long distance. This technique typically uses four coils as opposed to two coils used in conventional inductive links. In the four-coil system, the adverse effects of a low coupling coefficient between primary and secondary coils are compensated by using high-quality (Q) factor coils, and the efficiency of the system is improved. Unlike its two-coil counterpart, the efficiency profile of the power transfer is not a monotonically decreasing function of the operating distance and is less sensitive to changes in the distance between the primary and secondary coils. A four-coil energy transfer system can be optimized to provide maximum efficiency at a given operating distance. We have analyzed the four-coil energy transfer systems and outlined the effect of design parameters on power-transfer efficiency. Design steps to obtain the efficient power-transfer system are presented and a design example is provided. A proof-of-concept prototype system is implemented and confirms the validity of the proposed analysis and design techniques. In the prototype system, for a power-link frequency of 700 kHz and a coil distance range of 10 to 20 mm, using a 22-mm diameter implantable coil resonance-based system shows a power-transfer efficiency of more than 80% with an enhanced operating range compared to ~40% efficiency achieved by a conventional two-coil system.

Design and Optimization of Resonance-Based Efficient Wireless Power Delivery Systems for Biomedical Implants

Optical Waveguide Theory

We present experimental results showing that long-period gratings in photonic crystal fibers can be used as sensitive biochemical sensors. A layer of biomolecules was immobilized on the sides of the holes of the photonic crystal fiber and by observing the shift in the resonant wavelength of a long-period grating it was possible to measure the thickness of the layer. The long-period gratings were inscribed in a large-mode area silica photonic crystal fiber with a CO2 laser. The thicknesses of a monolayer of poly-L-lysine and double-stranded DNA was measured using the device. We find that the grating has a sensitivity of approximately 1.4nm/1nm in terms of the shift in resonance wavelength in nm per nm thickness of biomolecule layer.

https://orbit.dtu.dk/files/10711921/4.pdf

Photonic crystal fiber long-period gratings for biochemical sensing.

We present highly sensitive refractometers based on a long-period grating in a large-mode-area photonic crystal fiber (PCF). The maximum sensitivity is 1500 nm/refractive index unit at a refractive index of 1.33, to our knowledge the highest reported for any fiber grating. The minimal detectable index change is 2 x 10(-5). The high sensitivity is obtained by infiltrating the sample into the holes of the PCF to give a strong interaction between the sample and the probing field.

Highly sensitive refractometer with a photonic-crystal-fiber long-period grating

We present highly sensitive refractometers based on a long-period grating in a large mode area PCF. The maximum sensitivity is 1500 nm/RIU at a refractive index of 1.33, the highest reported for any fiber grating. The minimal detectable index change is $2\times 10^{-5}$. The high sensitivity is obtained by infiltrating the sample into the holes of the photonic crystal fiber to give a strong interaction between the sample and the probing field.

Highly sensitive refractometer with photonic crystal fiber long-period grating

We study the sensitivity of fiber grating sensors in the applications of strain, temperature, internal label-free biosensing, and internal refractive index sensing. New analytical expressions for the sensitivities, valid for photonic crystal fibers are rigorously derived. These are generally valid, and we identify a previously unaccounted term for temperature and strain sensing. It is shown that dispersion plays a central role in determining the sensitivity, and that dispersion may enhance or suppress sensitivity as well as change the sign of the resonant wavelength shifts. We propose a quality factor, Q, for characterizing long period gratings sensors.

Sensitivity of photonic crystal fiber grating sensors: biosensing, refractive index, strain, and temperature sensing

In this paper we present the first incorporation of a microstructured optical fiber (MOF) into biochip applications. A 16-mm-long piece of MOF is incorporated into an optic-fluidic coupler chip, which is fabricated in PMMA polymer using a CO2 laser. The developed chip configuration allows the continuous control of liquid flow through the MOF and simultaneous optical characterization. While integrated in the chip, the MOF is functionalized towards the capture of a specific single-stranded DNA string by immobilizing a sensing layer on the microstructured internal surfaces of the fiber. The sensing layer contains the DNA string complementary to the target DNA sequence and thus operates through the highly selective DNA hybridization process. Optical detection of the captured DNA was carried out using the evanescent-wave-sensing principle. Owing to the small size of the chip, the presented technique allows for analysis of sample volumes down to 300 nL and the fabrication of miniaturized portable devices.

Lars Henning Rindorf

Papers

Photonic crystal fiber long-period gratings for biochemical sensing.

Highly sensitive refractometer with a photonic-crystal-fiber long-period grating

Highly sensitive refractometer with photonic crystal fiber long-period grating

Sensitivity of photonic crystal fiber grating sensors: biosensing, refractive index, strain, and temperature sensing

Towards biochips using microstructured optical fiber sensors.