==== Notes ====
  * [[https://www.sciencedirect.com/science/article/abs/pii/S0925400500007218]]
  * {{ :ocr60.pdf | Whitepaper describing use of 9,10-Diphenylanthracene for oxygen detection}} 
  * The oxygen quenching is predicted by the Stern-Volmer equation, and changes in intensity or lifetime of the fluorescence can be monitored
  * Ruthenium complex
    * [[https://www.cyanagen.com/products/rubp3-pf62-ruthenium-complexes/]]
    * [[https://www.cyanagen.com/cyanacontent/uploads/Products/RuBP3-PF62/Documents/SDS/EN-IS_RuBP3-PF62-F3R050X_rev01.pdf]]
    * [[https://www.ruixibiotech.com/pts/ru-bpy3-pf6-2]] CAS: 60804-74-2 $400/g 
    * {{:rubp3-pf62_transient_absorption.png?direct&400|}}
  * [[https://www.eevblog.com/forum/projects/accurate-pulse-width-measurement/]]
  * [[https://www.ti.com/product/TDC7200]] 
    * Datasheet: [[https://www.ti.com/lit/ds/symlink/tdc7200.pdf]] 
    * [[https://github.com/Yveaux/TDC7200]]
  * {{ :quenching_of_the_fluorescence_of_tris_2_2_bipyridine_ruthenium.pdf |Quenching of the Fluorescence of Tris (2,2’-Bipyridine) Ruthenium(II),
[Ru(bipy)3]2+, by a Dimeric Copper(II) Complex}}

==== 390nm LED ====
  * [[https://www.digikey.com/en/products/detail/bivar-inc/UV3TZ-390-15/3095671]]

{{:ledpeak.png?direct&600|}}

==== Shortpass Filter / UV 400nm ====
  * [[https://www.asahi-spectra.com/opticalfilters/detail.asp?key=XHS0400]]

{{:uvshortpass.png?direct&600|}}

==== Fluorescent Target ====
  * 9,10-Diphenylanthracene
    * [[https://www.ebay.com/itm/256126158171]] 2.5G/$42
    * [[https://www.alibaba.com/product-detail/9-10-Diphenylanthracene-CAS1499-10-1_1601304899154.html]]

  * Absorption Peaks: 270nm, 354nm, 373nm, 393nm
{{:absorption.png?direct&600|}}

  * Fluorescence Emission Peaks: 404nm, 425m
{{:flourscence.png?direct&600|}}

  * rubrene (5,6,11,12-tetraphenylnaphthacene) is what's probably in the real sensor
    * excitation peak at 450 nm and an emission peak at 530 nm
    * [[https://www.alibaba.com/product-detail/OEM-factory-OLED-materials-CAS-517-1600157189956.html]]
    * [[https://www.sigmaaldrich.com/US/en/product/aldrich/554073]]

{{:rubeneabsorption.png?direct&600|}}
{{:rubenefluorescence.png?direct&600|}}

==== PDMS polymer binder ====
  * [[https://ocw.mit.edu/courses/hst-410j-projects-in-microscale-engineering-for-the-life-sciences-spring-2007/a8ccf98b4a870a05112f4143c5146c0a_manuf_pdms.pdf]]
  * Sylgard 184
    * [[https://www.amazon.com/dp/B0DHVM8ZGD]]
  * Sylcap 284 is available with smaller MOQ and has similar gas permeability but cures faster
    * [[https://www.amazon.com/MICROLUBROL-SYLCAP-Elastomer-Encapsulant-Transparent/dp/B074Z1MV3V]]

==== 400nm longpass filter ====
  * [[https://www.asahi-spectra.com/opticalfilters/detail.asp?key=XUL0400]]
{{:uvlongpass.png?direct&600|}}

==== Optical Sensor ====
  * Silicon photodiode covering as much UV as possible, make sure 420nm is in spectra
  * [[https://www.digikey.com/en/products/detail/vishay-semiconductor-opto-division/BPW21R/1681147?gQT=1]]
{{:photodiodespectra.png?direct&600|}}

===== Making Rubene Samples =====

==== Doping rubrene into PDMS ==== 
(e.g., Sylgard 184 or a similar PDMS variant like Sylcap 284).

Most published oxygen-sensor designs with rubrene start in a relatively narrow doping range to balance brightness and avoid dye aggregation or self-quenching.

==== 1. Typical Doping Range ====
  * **Starting Point**: **0.01–0.1 wt%** (by weight of rubrene relative to the total weight of uncured PDMS) is a commonly cited window.  
  * In practical terms, this might translate to about **0.1–1 mg of rubrene per gram of PDMS base**.  
  * If you see **aggregation** or **self-quenching** (the fluorescence drops because the molecules are packed too closely), reduce the concentration. If the **signal is too weak**, you can increase it slightly toward the upper end of the range.

=== Why This Range? ===

  - **Fluorescence Brightness vs. Self-Quenching**
    * Rubrene is highly fluorescent, so even a small concentration can yield significant emission.  
    * Above roughly **0.1–0.2 wt%**, many organic dyes (including rubrene) begin to exhibit “concentration quenching,” where the emission drops due to exciton–exciton annihilation or dye aggregation.

  - **Cost and Solubility**  
    * Rubrene can be expensive, and it’s only moderately soluble in typical organic solvents. Over-saturating the mix will result in precipitates, optical scattering, or inhomogeneous films.

  - **Oxygen Diffusion**  
    * PDMS is highly oxygen-permeable, which is great for oxygen sensing but also means the dye gets quenched quickly. Going too high in dye content doesn’t necessarily improve sensitivity—it may just cause more self-quenching without improving the sensor’s performance.

==== 2. Practical Mixing Tips ====

  - **Pre-Dissolve the Rubrene**  
    * Dissolve rubrene in a small volume of an organic solvent (toluene or chloroform) at a known concentration (e.g., 1–10 mg/mL).  
    * Slowly add this solution to the PDMS base resin while stirring. This helps achieve more uniform dispersal.

  - **Degas**  
    * After mixing, place the solution under vacuum to remove both solvent and air bubbles.  
    * If you have a two-part system (base + curing agent), you might do a preliminary degassing before adding the curing agent, then a final degassing after thoroughly mixing in the catalyst.

  - **Curing**  
    * Many PDMS systems can cure at room temperature in 24–48 hours or at 60–70 °C for a few hours.  
    * Try not to exceed 120 °C or keep the mixture at high temperature for too long—rubrene will degrade over time if both heat and oxygen are present.

==== 3. Considerations for a 450 nm LED ====

  - **Absorption Spectrum of Rubrene**  
    * Rubrene absorbs sufficiently at ~450 nm to fluoresce orange. 
    * Verifying the absorbance in your final PDMS film to ensure you’re not “under-absorbing.”

  - **Film Thickness**  
    * If the film is too thick or dye concentration is too high, inner regions might not be excited properly due to light attenuation.  
    * A thickness of **~0.1–1 mm** is typical for sensor films, but this depends on optical design and the LED intensity.

  - **Sensor Calibration**  
    * Once cured, measure the fluorescence intensity (and/or lifetime) under different known oxygen concentrations to build a calibration curve using the Stern–Volmer relationship.


==== 4. Adjusting the Ratio Over Time ====

  * **If the Film Is Too Dark (Self-Absorbing)**  
    * Lower the dye percentage. Even going from 0.1 wt% down to 0.02 wt% can make a big difference in clarity and reduce self-quenching.  
  * **If the Signal Is Too Dim**  
    * Increase the dye content in small increments (e.g., from 0.02 wt% up to 0.05 wt%), but watch for diminishing returns due to concentration quenching.

==== Bottom Line ====

A good **starting point** is **0.01–0.1 wt% rubrene** relative to the PDMS base. Begin on the lower side (e.g., **0.02–0.05 wt%**) to avoid aggregation, see how bright the sensor is at 450 nm excitation, and adjust accordingly. Once you have a homogeneously dispersed rubrene-PDMS film, you can calibrate its oxygen response using the Stern–Volmer equation and fine-tune the formulation to optimize fluorescence intensity versus oxygen quenching sensitivity.

==== pH optode sensor notes ====
  * {{ :nandi-amdursky-2022-the-dual-use-of-the-pyranine-_hpts_-fluorescent-probe-a-ground-state-ph-indicator-and-an-excited_1_.pdf | Pyranine (HPTS) Fluorescent Probe}}
  * Pyranine Solvent Green 7 CAS 6358-69-6 aka HPTS aka Pyranine
  * {{ :msds_pyranine_h1529.pdf | MSDS: Solvent Green 7}}
  * [[https://www.alibaba.com/product-detail/Haihang-Industry-Pyranine-Solvent-Green-7_1601194307391.html]]
  * [[https://www.amazon.com/Eastchem-Solvent-Green-CAS-6358-69-6/dp/B087FF2TYQ]]
  * Tetraethyl orthosilicate (TEOS) or methyltriethoxysilane (MTES)
  * {{ :art3a10.11342fs0020168507090178.pdf |Preparation of sols from water-alcohol solutions of tetraethyl orthosilicate and SnCl4 and the effect of sol composition on the surface morphology of sol-gel films}}
  * {{ :influence_of_water_molar_ratio_on_fabrication_of_s.pdf |Influence of Water Molar Ratio on Fabrication of Silica Ceramic Membranes via Sol-Gel Dip-Coating Method}}
  * [[https://www.chemedx.org/blog/ph-sensitive-highlighter-%E2%80%9Cflame%E2%80%9D]]