3D printed O2 indicators.

A polymer filament, containing an O2-sensitive lumophore pigment, platinum(ii) (pentafluorophenyl) porphyrin coated onto nanoparticulate silica particles, i.e. PtTFPP/SiO2, is produced and used in a filament which is then 3D printed as O2-sensitive indicator dots (1 cm diameter, 30 µm thick) on a polyethylene terephthalate (PET) supporting film substrate. Two different filaments, prepared using the polymers, low density polyethylene (LDPE) and polylactic acid (PLA), respectively, are used to produce 3D printed PtTFPP/SiO2/LDPE and PtTFPP/SiO2/PLA O2-sensitive indicator dots, with dynamic ranges of 0-30% and 0-400% O2, respectively. The O2 response characteristics of these two, very different, indicator dots, such as sensitivity, response time and temperature sensitivity, are measured and compared with those of a commercial O2 indicator, FOSPOR (OceanInsight) and other commercial O2 indicators. The potential of this method for mass manufacture of O2 indicator dots and the likelihood that it can be extended to produce other optical indicators, such as those for ammonia or CO2, are discussed briefly.

A colourimetric vacuum air-pressure indicator.

A colourimetric vacuum air pressure indicator is described, based on the very low level of CO2 in air. The indicator uses the pH indicator dye, ortho-cresolphthalein, OCP, which is violet coloured in its deprotonated form and colourless when protonated. When the violet coloured OCP anion is ion-paired with the tetrabutylammonium cation, the product is readily dissolved in a non-aqueous solution containing the polymer ethyl cellulose to create an ink which, when cast and allowed to dry, responds to levels of CO2 well below that in air, i.e.≪0.041%; the indicator's halfway colour changing point is at 0.062 atm of air at 22 °C, which is interesting in that in food vacuum packaging the pressure in the pack is usually ca. 0.04 atm. The indicator can be used as a qualitative and quantitative indicator of vacuum air pressure. The latter requires the use of digital photography, coupled to RGB colour analysis, in the analysis of the indicator's colour. As with most CO2 indicators, the indicator's response is temperature sensitive, with ΔH = 78 ± 5 kJ mol-1. The indicator's 90% response and recovery times to a cycle of vacuum and air are 16.2 and 2.7 min, respectively. The efficacy of the indicator as a vacuum-package integrity indicator for food packaging is illustrated and other potential applications are discussed briefly. This is the first reported example of an ink-based, inexpensive, colourimetric vacuum air pressure indicator.

Highly CO2 sensitive extruded fluorescent plastic indicator film based on HPTS.

Highly-sensitive optical fluorescent extruded plastic films are reported for the detection of gaseous and dissolved CO2. The pH-sensitive fluorescent dye used is 8-hydroxypyrene-1,3,6-trisulfonic acid trisodium salt (HPTS, PTS(-)) coated on the surface of hydrophilic fumed silica and the base is tetrabutylammonium hydroxide (TBAH). The above components are used to create an HPTS pigment (i.e. HPTS/SiO2/TBAH) with a high CO2 sensitivity (%CO2 (S = 1/2) = 0.16%) and fast 50% response (t50↓) = 2 s and recovery (t50↑) = 5 s times. Highly CO2-sensitive plastic films are then fabricated, via the extrusion of the HPTS pigment powder in low-density polyethylene (LDPE). As with the HPTS-pigment, the luminescence intensity (at 515 nm) and absorbance (at 475 nm) of the HPTS plastic film decreases as the %CO2 in the ambient gas phase increases. The HPTS plastic film exhibits a high CO2 sensitivity, %CO2 (S = 1/2), of 0.29%, but a response time <2 min and recovery time <40 min, which is slower than that of the HPTS pigment. The HPTS plastic film is very stable under ambient conditions, (with a shelf life >six month when stored in the dark but under otherwise ambient conditions). Moreover, the HPTS-LDPE film is stable in water, salt solution and even in acid (pH = 2), and in each of these media it can be used to detect dissolved CO2.

Early wound infection monitoring via headspace O2 micro-respirometry.

A luminescence based, inexpensive, 3D printed O2 indicator is incorporated into a commercial, clear, occlusive wound dressing, which allows the %O2 in the headspace above a simulated wound to be monitored. Two wound models are used to evaluate this micro-respirometry-based system for monitoring wound infection namely, a simple 'agar plug' model and a wounded porcine skin model. Inoculation of either wound model with E. coli, E. cloacae, or A. baumannii, produces the typical 'S'-shaped, τ vs incubation time, t, profiles, associated with micro-respirometry, due to the decrease in %O2 in the headspace above the wound. A threshold value for the lifetime, τTT, of 21.1 µs, is identified at which the bacterial load is equal to the critical colonization threshold, CCT, ca. 106 colony forming units, CFU/mL, above which infection is highly likely. The agar plug wound model/O2 indicator combination is used to identify when the CCT is reached for a wide range of inoculant concentrations, spanning the range 108-101 CFU/mL, for all three microbial species. The O2 indicator is also successfully evaluated using a porcine skin wound model inoculated with E. coli. The results of this work are compared to other reported, usually invasive, smart wound monitoring systems. The possible use of this new, non-invasive smart-wound dressing technology, both at the point of care and at home, are discussed briefly.

Colorimetric CO2 Indicators.

Carbon dioxide, CO2, is an essential part of life, in that through green plant photosynthesis it is used to generate food and fuel and is generated in both aerobic and anaerobic respiration. Industrially, it is used in fire extinguishers, supercritical fluid extractions, and food packaging. Environmentally, it is in the atmosphere, hydrosphere, and biosphere and is responsible for global warming and the acidification of the oceans. The monitoring of CO2 in the gas phase is usually carried out using FTIR spectroscopy, whereas the measurement of dissolved CO2 usually involves an electrochemical device. Excitingly, the most recent forms of CO2 indicators appear to offer significant advantages over current methods, such as simplicity, low cost, and portability. This Account highlights the work of the Mills group on transforming CO2 colorimetric indicator technology from the usual water-based (i.e., "wet") indicator form to dry CO2-sensitive inks, pigments, plastics, and adhesives. Initially, the basic theory associated with colorimetric CO2 indicators is described, and the simple relationship between indicator absorbance and the partial pressure of CO2, PCO2, established. The early work on CO2-sensitive inks is then described, where such inks comprise a hydrophilic pH-sensitive dye anion, coupled with a lipophilic quaternary ammonium cation, dissolved in a nonaqueous solution of a polymer which, when cast, forms a dry ink film that gives a reversible color response when exposed to CO2 both in the gas phase and dissolved in solution. The ability to tune the sensitivity of a CO2 ink film to the desired application through the judicious choice of the pH indicator dye and base concentration is described. The dependence of the sensitivity of a CO2 ink film on temperature is used to create a temperature indicator, and the ability to tune the ink, to respond to high levels of CO2, is used to create a fizziness indicator for carbonated drinks. Very sensitive CO2 inks are used to make a vacuum and a general air-pressure indicator. The more recent development in CO2 indicator technology is described in which CO2 inks are used to coat silica particles to make a range of different CO2-sensitive pigments, which, when incorporated into a plastic, through extrusion, produce a range of novel CO2-sensitive plastic films that have many notable advantages over their ink film counterparts. Examples are then given of such plastic films being used for dissolved CO2 measurements in salt water, for food packaging, and as an early wound-infection indicator. Finally, the recent incorporation of a CO2-sensitive pigment into a pressure sensitive adhesive to make an after opening freshness tape is described briefly. Although most commercial CO2 indicators are assessed by eye and so are limited to qualitative analysis, this work shows that colorimetric CO2 indicators can be used for quantitative analysis through absorbance measurements. Nowadays, such measurements can be readily made using just a digital camera and color analysis software via digital camera colorimetry, DCC, which is likely to have a significant impact on the widespread use of the CO2 indicators described herein, their commercial viability, and their potential areas of application.

Non-invasive, 3D printed, colourimetric, early wound-infection indicator.

A simple, non-invasive, colour-based carbon dioxide (CO2) indicator is described. The indicator provides an indirect response to the rapid, aerobic microbial colonisation of an underlying wound when used in conjunction with an occlusive (i.e. sealed) dressing. The indicator has potential as an early warning indicator of infection in chronic wounds.

Colourimetric plastic film indicator for the detection of the volatile basic nitrogen compounds associated with fish spoilage.

The first example of an extruded polymer film containing the pH sensitive dye bromophenol blue (BPB) is described in which the polymer encapsulated dye changes colour from yellow to blue upon exposure to basic volatile nitrogen compounds, such as those given off by fish as it spoils. The latter include: trimethylamine (TMA), dimethylamine (DMA) and ammonia (NH3), and are collectively known as total volatile basic nitrogen (TVB-N). The films' colourimetric response to specific levels of TMA, as measured using absorbance spectroscopy and digital photography coupled with RGB colour analysis, is reported. The indicator is then used as a fish spoilage indicator at 22 and 4 °C, whilst at the same time a microbiological study is carried out, and in both cases the results reveal a strong correlation between the change in colour of the indicator with the concentration of bacterial colony forming units on the fish; the latter is often used as a measure of fish freshness. The correlation arises because the increase in TVB-N in head space of the package is due to the gradual bacteria-induced decomposition of the fish. The colourimetric TVB-N plastic film indicator's potential as a spoilage indicator for packaged fresh fish is discussed briefly.