Year: 2021

Xiujun Li’s group at the University of Texas at El Paso recently published an article in Analytical Chemistry that builds on the “V-chip” concept from a 2012 Nature Communications publication by the Qin group. The chip uses the position of a visible liquid in a channel to allow the user to quantitatively determine the concentration of an analyte of interest by simple visual inspection – no detector needed!

The chip (schematic at right) operates similarly to an alcohol or mercury thermometer in which the user reads the liquid meniscus level against a scale to determine the temperature. In the case of these microfluidic V-chip devices, the liquid level moves in response to the concentration of an analyte. In the seminal paper by Qin in 2012, the liquid plug was driven by gaseous products generated in a reaction with the analyte. A sandwich ELISA (enzyme-linked immunosorbent assay) reaction was used in which the unbound antibody was labelled with nanoparticles conjugated to a catalase enzyme. At the end of the ELISA reaction, the bound catalase labels reacted with preloaded hydrogen peroxide in the solution to produce oxygen gas in proportion to the amount of analyte initially present. The liquid ink was driven along connected ‘thermometer’ microfluidic channels and measured accordingly to determine the concentration of analyte. This volumetric relationship gives rise to the V in V-chip.

In the work of Li’s group, a sandwich ELISA is also used, but the labelling and gas generation are different, and arguably improved. Instead of generating gas enzymatically with catalase, it is generated photothermally with a near infrared (NIR) laser. In their work, the analyte of interest is the prostate cancer biomarker PSA (prostate-specific antigen), and the unbound antibody is labelled with an Fe₃O₄ nanoparticle. The nanoparticle is chemically converted to a Prussian Blue (PB) nanoparticle which is a strong NIR-absorbing photothermal agent. At the end of the ELISA reaction, an NIR laser is shone onto the sample chamber and the incident radiation is converted by the PB nanoparticles to heat and generates vapour, causing liquid to be driven along the thin radiating ‘thermometer’ channels. The extent of displacement of the liquid again depends directly on the amount of PB nanoparticle-labelled antibodies present, which is a function of the concentration of PSA to which they bind. An example with blue dye showing the liquid after ELISA reaction and photothermal interrogation is shown above right.

The V-chips in both studies offer the huge benefit of not requiring any measuring instrumentation for detection; no optical or electrochemical detector, signal processing and display required. With a properly designed microfluidic chip and calibrated scale, the results of an ELISA-based cancer biomarker assay can be read directly from the chip like a thermometer. The dramatic reduction in complexity and cost can be a huge gain in a derived product concept. In addition, Li’s work uses a photothermal heating process in lieu of the second enzymatic reaction (catalase) in the analysis chain, which both simplifies the procedure, and is likely to make it more robust, since photothermal heating is more controllable and reproducible than an enzymatic reaction. The cost for this is an added on- or off-board NIR laser and lens, both of which can be mass-fabricated cheaply. This approach potentially improves analytical performance, which could in turn be an enabling piece of the technology foundation for a related product.

As both a proud and loving owner of a loyal golden doodle (Fezzik) and an analytical chemist, I couldn’t resist this one! I also can’t resist posting a photo of our family’s lovable hot dog burglar, Fezzik. He’s in his winter attire in this photo from last Saturday morning’s breezy -34°C walk. He’s amazingly well adapted to the cold, with enough circulation to keep his exposed nose and thin ears nice and warm no matter what!

But I digress. Did you know that man’s best friend is an analytical chemistry superhero? Well, I guess we are all familiar with the notion that dogs have incredibly sensitive noses. We may have seen handlers harnessing their dog’s superpower for good, such as with drug sniffer dogs at airports and accelerant sniffer dogs at arson crime scene. A dog/hander team photo of German Sheppard Ezra and Jeff Lunder of CADA Fire Dogs from the Harynuk paper described below is shown below.

It should perhaps come as no surprise that there’s some really cool biology behind their abilities. Chemist and founder of Chemistry Matters Dr. Court Sandau recently posted a link on LinkedIn to an excerpt of a talk he gave describing the special design of canine olfactory systems – have a look. Dogs have 300 million sensing cells (~50x as many as humans), direct over 10% of their inhaled air over these sensors, and can expand the area at will to allow for expanded sensitivity … absolutely fascinating!

It may come as a surprise, however, to learn that their olfactory power is more sensitive than the best analytical chemistry methods and instrumentation, and that this actually causes us problems as they work their magic. The dilemma arises when a forensic lab cannot detect the presence of an accelerant that a dog very likely correctly identified, due to the lab’s inferior limit of detection (LoD, the lowest amount or concentration for which something can be detected by a dog, detector, etc.); such a situation can potentially render arson evidence inadmissible.

A paper published in Forensic Chemistry last year by Professor James Harynuk’s group at the University of Alberta tackled the first step in addressing the gap between canine and analytical instrumentation/methodology performance: determining the canine limit of detection for an accelerant. With an idea of dogs’ capabilities, analytical chemists would at least know the target they are trying to hit!

A great deal of effort was required by Harynuk’s group to conceive of and validate methods to clean and prepare slate tile substrates to receive accelerant samples; the extent of initial contaminants and cleaning effectiveness was demonstrated with solid phase microextraction followed by gas chromatographic – mass spectrometry headspace analysis. They also had to research and chose suitable solvents and stabilisers in which to prepare diluted accelerant solutions. Taking great care to eliminate any source of bias for the dog and handler during the identification trials, Harynuk’s team applied an increasingly diluted range of accelerants such as lighter fluid, regular gasoline and diesel gasoline, to slate tile substrates located amongst blank and control tile samples with other petroleum components typical of a scene. An example of a sample being applied to a tile substrate is shown at right. For the trials, two dog/handler teams were brought in to determine their ability to correctly identify the spiked tile from the group, and at increasingly weaker doses. Both dog/handler teams were able to correctly detect tiles spiked with as little as 5 pL of gasoline (1 pL is one billionth of 1 mL)!

And so now we know why it is that, when we quietly peel the film wrap off steaks to be prepared for the grill, our dog smells it from the other end of the house, two stories up, and comes running with eyes full of hope and obedience … before the first dash of steak spice has hit the meat!

A recent technical note in Analytical Chemistry from Professor Carolyn Ren’s lab at the University of Waterloo demonstrates an efficient and controlled approach for on-demand microfluidic droplet generation.  The technique is used in a variety of important bio-medical and industrial applications such as: single or multiple cell sorting, culturing and incubation; droplet-based PCR and DNA sequencing; and chemical synthesis including micro- and nanoparticle synthesis.

Microfluidic droplet generation at a tee intersection on a chip can be initiated or controlled by several approaches or factors such as balancing input pressures, channel intersection geometry, and surface tension at the interface.  The latter offers an opportunity for fine control: input pressures for immiscible carrier and droplet fluids are nearly matched, and alteration of the surface tension and thus Laplace pressure at the interface meniscus will lead to on-demand droplet generation.  Surface tension is a function of the chemical composition of the carrier and droplet fluids as well as temperature, so localised heating at the meniscus can enable fine control of droplet generation.  On-chip resistive thermal heating works well, but has a slow response; laser cavitation is much faster, but requires expensive, delicate optically aligned off-chip instrumentation.  The authors’ choice of on-chip microwave heating, shown at right above, has very rapid response times, is implemented with very simple (and cost-effective) microfabricated heating resonator electrodes, and affords precisely localised heating based on electrode design and chemically selective absorption of microwave energy by the aqueous droplet fluid (and not the carrier oil or PDMS device material).

Time-lapsed photographs of droplet generation of water (vertical channel) in carrier oil (horizontal channel) using their system are shown at right below; the curved black line is the electrode under the vertical channel.  The research showed a relationship between applied microwave power and generation time, but with droplet sizes remaining at a constant ~1.8 nL in their 40 µm deep x 210 µm wide channels.

The advantage of repeatable, precisely controlled on-demand droplet generation for the many fields in which droplet-based microfluidics is used could be important.  The fact that it should be manufacturable from different microfluidic device materials without expensive instrumentation is a considerable advantage as well.  The localised nature of the heating could also be beneficial in thermally sensitive assays where excessive and/or prolonged heating is detrimental.