Monolithic optofluidic chips: from optical manipulation of single cells to quantum sensing of fluids

This paper was presented at the 4th Micro and Nano Flows Conference (MNF2014), which was held at University College, London, UK. The conference was organised by Brunel University and supported by the Italian Union of Thermofluiddynamics, IPEM, the Process Intensification Network, the Institution of Mechanical Engineers, the Heat Transfer Society, HEXAG - the Heat Exchange Action Group, and the Energy Institute, ASME Press, LCN London Centre for Nanotechnology, UCL University College London, UCL Engineering, the International NanoScience Community, www.nanopaprika.eu.We report on a new class of integrated optofluidic devices, fabricated by femtosecond laser micromachining. The capability to combine optical waveguides with microfluidic channels in the same glass chip provides a very powerful platform, introducing new tools in the field of optical sensing. Two recent applications that greatly benefitted from this novel technology are on-chip optical manipulation of single cells by optical forces and optical sensing of the refractive index of fluids by quantum states of light. The specific properties of robustness, alignment free and portability of these devices pave the way to the use of these advanced sensing technologies outside the lab, in a real application environment

A review of single-mode fiber optofluidics

We review the field we describe as “single-mode fiber optofluidics” which combines the technologies of microfluidics with single-mode fiber optics for delivering new implementations of well-known single-mode optical fiber devices. The ability of a fluid to be easily shaped to different geometries plus the ability to have its optical properties easily changed via concentration changes or an applied electrical or magnetic field offers potential benefits such as no mechanical moving parts, miniaturization, increased sensitivity and lower costs. However, device fabrication and operation can be more complex than in established single-mode fiber optic devices

Optical imaging techniques in microfluidics and their applications

Microfluidic devices have undergone rapid development in recent years and provide a lab-on-a-chip solution for many biomedical and chemical applications. Optical imaging techniques are essential in microfluidics for observing and extracting information from biological or chemical samples. Traditionally, imaging in microfluidics is achieved by bench-top conventional microscopes or other bulky imaging systems. More recently, many novel compact microscopic techniques have been developed to provide a low-cost and portable solution. In this review, we provide an overview of optical imaging techniques used in microfluidics followed with their applications. We first discuss bulky imaging systems including microscopes and interferometer-based techniques, then we focus on compact imaging systems that can be better integrated with microfluidic devices, including digital in-line holography and scanning-based imaging techniques. The applications in biomedicine or chemistry are also discussed along with the specific imaging techniques

Integrated collinear refractive index sensor with Ge PIN photodiodes

Refractive index sensing is a highly sensitive and label-free detection method for molecular binding events. Commercial implementations of biosensing concepts based on plasmon resonances typically require significant external instrumentation such as microscopes and spectrometers. Few concepts exist that are based on direct integration of plasmonic nanostructures with optoelectronic devices for on-chip integration. Here, we present a CMOS-compatible refractive index sensor consisting of a Ge heterostructure PIN diode in combination with a plasmonic nanohole array structured directly into the diode Al contact metallization. In our devices, the photocurrent can be used to detect surface refractive index changes under simple top illumination and without the aid of signal amplification circuitry. Our devices exhibit large sensitivities > 1000 nm per refractive index unit in bulk refractive index sensing and could serve as prototypes to leverage the cost-effectiveness of the CMOS platform for ultra-compact, low-cost biosensors.Comment: 21 pages, 6 figures, supporting information with 11 pages and 11 figures attache

Photonic crystal resonator integrated in a microfluidic system

We report on a novel optofluidic system consisting of a silica-based 1D photonic crystal, integrated planar waveguides and electrically insulated fluidic channels. An array of pillars in a microfluidic channel designed for electrochromatography is used as a resonator for on-column label-free refractive index detection. The resonator was fabricated in a silicon oxynitride platform, to support electroosmotic flow, and operated at 1.55 microns. Different aqueous solutions of ethanol with refractive indices ranging from n = 1.3330 to 1.3616 were pumped into the column/resonator and the transmission spectra were recorded. Linear shifts of the resonant wavelengths yielded a maximum sensitivity of 480 nm/RIU and a minimum difference of 0.007 RIU was measured

Advances in Optofluidics

Optofluidics a niche research field that integrates optics with microfluidics. It started with elegant demonstrations of the passive interaction of light and liquid media such as liquid waveguides and liquid tunable lenses. Recently, the optofluidics continues the advance in liquid-based optical devices/systems. In addition, it has expanded rapidly into many other fields that involve lightwave (or photon) and liquid media. This Special Issue invites review articles (only review articles) that update the latest progress of the optofluidics in various aspects, such as new functional devices, new integrated systems, new fabrication techniques, new applications, etc. It covers, but is not limited to, topics such as micro-optics in liquid media, optofluidic sensors, integrated micro-optical systems, displays, optofluidics-on-fibers, optofluidic manipulation, energy and environmental applciations, and so on

Materials and methods for modular microfluidic devices

This thesis work concerns the investigation of materials and methods that can be applied to the realization of microfluidic devices (MFDs). In particular, the attention is placed on modular MFDs, as opposed to fully integrated ones. The reasons behind this choice are given in detail in Section 1.2 of this work, but they can be here summarized in the fact that while integrated MFDs offer great advantages in terms of portability, modular devices are more versatile, and so particularly well suited for research applications. The first part of the work here reported describes the microfabrication techniques employed for the realization of single-function microfluidic modules. Devices have been fabricated through PDMS replica molding from SU-8 masters. Masters have been in turn realized through masked UV-lithography or one- or two-photon direct laser writing, depending on the resolution requirements. The replica molding method is a very fast and efficient way to realize MFDs, but suffers from some limitations in the structure shapes that can be successfully replicated. In light of this, a photopolymerizable hybrid organic/inorganic sol-gel blend is proposed and tested as alternative material for MFDs fabrication. The characterization results reveal that this material is biocompatible and features better mechanical properties than PDMS, but structures with more than one dimension exceeding a few micrometers tend to crack during fabrication, making this blend unusable as bulk material. Still, this material could be efficiently employed to fabricate sub-structuration inside PDMS channels. Following this investigation on materials, a microfluidic mixing module is proposed and tested. Since laminar flow conditions dominate inside microchannels, efficient mixing in MFDs require the use of specifically designed mixers. The proposed module makes use of obstructions inside a microchannel to perturb the laminar flow and thus enhance mixing of two species. The most efficient geometries have been selected with the aid of numerical simulations, and two promising layouts have been fabricated and experimentally tested by measuring the dilution of a fluorophore (mixing between a fluorophore solution and pure solvent) through confocal fluorescence microscopy. Thirdly, the fabrication and characterization of an optofluidic light switching module is reported. This device employs a water/air segmented flow generated by a T-junction to alternatively transmit or total-reflect a laser beam. This deflection is proved to be periodical, and its frequency can be varied nonlinearly by adjusting the injection flow rates of air and water. The duty cycle of the module is also characterized, and a method to modulate it by increasing the water temperature is proposed and verified. Finally, a number of attempts to generate a nanoporous, low refractive index PDMS are described. The identification of an efficient procedure to fabricate this kind of material would lead to the possibility of using common microfluidic channels as water-core waveguides. To date, these attempts have not been totally successful, but critical points are identified, and viable strategies for future works on the subject are proposed