Thymol blue

MM supplemented with the indicated carbon source (0.5%, wt/vol) as well as a mixture of the pH indicators thymol blue (0.06 g/liter; Sigma) and bromothymol blue (0.06 g/liter; Sigma) was dispensed into a 384-well plate (30 µl) and inoculated with the indicated strain (10 µl). The OD₆₁₅ was measured every 30 min for 18 h to monitor the rate of sugar transport as previously described (11 (link)). In parallel, growth in MM without pH indicators was monitored by measuring the OD₆₅₅.

Bromphenol blue, bromocresol green, thymol blue, methyl red, nitrazine yellow, methyltriethoxysilane, triethoxy(octyl)silane, 2-methoxyethanol, propylene glycol methyl ether acetate, polyethylene glycol tert-octylphenyl ether, pararosaniline base, N,N-dimethyl-4,4’-azodianiline, p-toluenesulfonic acid and sulfuric acid were purchased from Sigma-Aldrich (Arklow, Co. Wicklow, Ireland). Analytical standards were also purchased from Sigma-Aldrich, Ireland. Substrates (Polygram® CEL 300) were obtained from Machery-Nagel GmbH (Düren, Germany). Water used was high purity Milli-Q water (Millipore >18 MΩcm). A vacuum sealer (iLmyh, model number ZS-11W2) and vacuum sealer bags (Culivac, B16025S) were used for sensor storage and shipment.

Branched
PEI (average M_w 25k g·mol^–1, ≤1% water), poly(4-styrenesulfonic
acid) (PSS, average M_w 75k g·mol^–1, 18 wt % in water), ammonia (NH₃, for
analysis EMSURE ISO, Reagent. Ph Eur, 25% in water), dimethylamine
solution [(CH₃)₂NH, 40 wt % in H₂O], sodium hydroxide (NaOH, >98% pellets), ethanol (absolute,
≥99%),
DMSO (ACS reagent, ≥99.9%), and thymol blue (ACS reagent) were
purchased from Sigma-Aldrich (The Netherlands). A 64.9 wt % TiO₂ particle suspension in water (average diameter ∼ 260
nm, determined by dynamic light scattering) was received from Akzo
Nobel Coatings BV, Sassenheim, The Netherlands. All chemicals were
used as received. All water used was deionized water (Milli-Q, Merck,
The Netherlands). Acetate sheets (250 μm thick) were purchased
from JEJE Produkt, The Netherlands.

CLEVIOS^TM PH 1000 suspension (PEDOT:PSS) was purchased from Heraeus (Hanau, Germany). (3-glycidyloxypropyl)trimethoxysilane (GOPS), sodium dodecylbenzenesulfonate, potassium nitrate, potassium hydroxide, sodium hydroxide, nitric acid, boric acid, hydrochloric acid, sodium dihydrogen phosphate, agarose, ammonia (28–30%), glycerol (≥99.5%), methyl red, phenolphthalein, thymol blue, bromothymol blue, and methylene blue were purchased from Sigma Aldrich (St. Louis, MO, USA). Ethylene glycol (EG) was obtained from Carlo Erba (Milano, Italy). Iridium tetrachloride (IrCl₄, powder) was purchased from Alfa Aesar (Ward Hill, MA, USA). Silicone elastomer and curing agent for preparation of PDMS were obtained from Sylgard (Dow Europe, Weisbaden, Germany). All chemicals were of reagent grade or higher.

All reagents were Analytical Reagents grade. m-cresol purple (1), o-cresol red (2), bromothymol blue (3), thymol blue (4), chlorophenol red (5), Ellman’s reagent (6) were purchased from Carlo Erba or Sigma Aldrich.
Dylon Colour Catcher^® was bought in a local supermarket.
Beast poultry meat in slices, sliced beef, pork and cod fillets were bought in a local supermarket (UNES Supermarkets, via Fratelli Cervi, 11 27100 Pavia, Italy), the same day of the delivery from the central slaughterhouse or fish market, a few moments after the food was put on the shelf.
A Smartphone Samsung Galaxy S7 was used to acquire pictures of the array; a portable led light box was used to guarantee the reproducibility of the photos. We also used a Puluz Photography Lightbox, Shenzhen Puluz Technology Limited, already shown elsewhere [12 ].
In this study we employed the sensor array already proposed in a previous work for chicken meat degradation monitoring [12 ]; the sensitive part, the solid support and the experimental setup were kept unchanged, but we extended its applicability towards different food samples.

pH was measured in the larval midgut using Thymol blue, m-Cresol purple, Cresol red, Bromocreosol purple, BromoThymol blue, Phenol Red and Congo red indicator dyes (all purchased from Sigma- Aldrich). Indicators were added to melted Drosophila diet (0.1% w/v), immediately mixed, and allowed to cool to room temperature. Larvae of the appropriate genotype were added, and after 2 hours the midgut excised in Schneider’s insect medium (Invitrogen). Micrographs were taken immediately using a Sony CyberShot NEX-C3 mounted on a Leica stereo microscope, as pH remains stable for only a few minutes after dissection. Images were processed using Adobe Photoshop CS5.1. Where noted, the diet was also supplemented with 1 mM Omeprazole (Sigma-Alrdich) or 100 μM acetazolamide (Sigma-Aldrich).