Thiol-Functionalized Capture Substrate with In-Situ MAPbBr₃ Fluorescence Readout for Field-Deployable Parts-Per-Billion Lead Detection in Potable Water

Inventor record / pre-art-search disclosure, v1 Date: 2026-04-21 Status: DRAFT, not filed. Internal technical disclosure prepared for patent-attorney review and prior-art search.

1. Abstract

Disclosed is a field-deployable cartridge and method for detecting dissolved lead in potable water at parts-per-billion (ppb) and sub-ppb concentrations without instrumentation beyond a handheld 365 nm UV source. A bed of (3-mercaptopropyl)trimethoxysilane (MPTS) functionalized silica captures Pb²⁺ from flowing water through thiolate chelation (HSAB soft–soft pairing, Pb–S log K ≈ 17). Capture efficiency at trace concentrations is increased by either a Tesla valvular conduit (, Tesla 1920) operated in a captive recirculation loop, or a throughput spigot-mounted cartridge in which a metered volume of tap water passes the bed over a defined cycle. Upon cycle completion, the substrate is sprayed with methylammonium bromide (MABr) dissolved in isopropanol. MABr reacts with captured Pb²⁺ in situ to crystallize methylammonium lead bromide (MAPbBr₃) perovskite at each binding site, producing a localized bright-green fluorescence (λ_em ≈ 520 nm, FWHM ~20 nm) under 365 nm UV excitation. The glow is visually correlated to captured Pb mass. The combination of thiolate preconcentration, Tesla-diodic fluidics, and in-situ perovskite formation on a disposable substrate yields a consumer-grade lead-in-water test with sensitivity previously available only in centralized laboratories.

2. Background and Prior Art

2.1 Thiol-functionalized silica for heavy-metal capture

Mercapto-silica sorbents have been studied for four decades as solid-phase extraction (SPE) media for soft heavy metals. MPTS, (3-mercaptopropyl)trimethoxysilane, grafted onto silica gel or mesoporous silica (MCM-41, SBA-15) via aqueous or toluene-based silanization yields a surface presenting free thiol groups at loadings of 0.8–2.5 mmol SH per gram of support, depending on support surface area and grafting conditions [Feng et al., Science, 1997, verify; Mercier & Pinnavaia, Adv. Mater., 1997, verify]. The thiolate is a soft Lewis base; Pb²⁺, Hg²⁺, Cd²⁺, and Ag⁺ are soft Lewis acids. The Pb–thiolate complex has an apparent binding constant log K ≈ 17 in aqueous media [Pearson, JACS, 1963, HSAB framework, verify]. This selectivity is exploited commercially in preconcentration cartridges upstream of ICP-MS and AA analyses. Prior art describes MPTS columns for analytical preconcentration but not, to the inventor's knowledge, for an integrated visual-readout consumer assay.

2.2 MAPbBr₃ perovskite fluorescence as a Pb²⁺ indicator

Methylammonium lead bromide (CH₃NH₃PbBr₃, MAPbBr₃) is a hybrid organic–inorganic perovskite that forms spontaneously upon mixing MABr with a soluble Pb²⁺ source in a polar aprotic or alcoholic solvent. The product emits sharply peaked photoluminescence at 515–525 nm under UV (≤ 400 nm) excitation, with photoluminescence quantum yields of 10–90 % depending on morphology and passivation [Pellet et al., Nature, 2018, verify]. Helmbrecht and co-workers demonstrated that MABr-laden paper or test strips, when contacted with surfaces bearing free Pb²⁺ (including weathered lead-based paint and leaded ceramics), produced immediate green fluorescence, the bright-green emission being diagnostic for Pb²⁺ specifically, as other common heavy-metal cations (Cu²⁺, Zn²⁺, Cd²⁺, Fe³⁺) do not form the 3-D perovskite phase with MABr and bromide at room temperature and therefore do not produce the same emission [Helmbrecht et al., Environmental Science & Technology, 2023, verify]. This Pb-specific perovskite signal has been commercialized (e.g., Lumetallix) as a surface-swab indicator but not, to the inventor's knowledge, integrated with aqueous capture-and-concentrate upstream chemistry for trace dissolved-lead detection.

2.3 Tesla valvular conduit

Nikola Tesla's valvular conduit [Tesla's 1920 valvular conduit. Modern microfluidic reductions of the Tesla design (e.g., Thompson et al., Journal of Fluids Engineering, 2014, verify; Nguyen et al., Micromachines, 2021, verify) measure diodicity ratios of 1.5 to > 2 across a range of Reynolds numbers. Tesla valves are used in micropumps, fuel-cell purge lines, and reactor-loop check-flow applications. Their use as a residence-time enhancer upstream of a sorbent bed, or as the asymmetric element in a captive recirculation loop intended to drive a near-closed-system sample past a sorbent repeatedly without a mechanical check valve, appears to the inventor to be novel in the context of water quality analysis.

2.4 Existing consumer lead-in-water tests

The consumer lead-in-water testing market is dominated by three categories of product:

1. Colorimetric test strips (rhodizonate, sodium sulfide, dithizone derivatives). Limit of detection (LoD) is typically 5–15 ppb with poor selectivity and visual ambiguity at or near the EPA action level of 10 ppb (EPA drinking-water Lead and Copper Rule, 40 CFR Part 141). False positives from iron, copper, and turbidity are common. Read windows are narrow (30–120 s).

1. Mail-in laboratory kits using graphite-furnace atomic absorption (GFAA) or ICP-MS. These offer LoD well below 1 ppb and strong regulatory credibility, but require shipping, incur multi-day delays, and cost $20–$60 per sample.

1. Handheld X-ray fluorescence (XRF) instruments are not suitable for dissolved lead, they measure surface or bulk elemental composition, not sub-ppm aqueous concentrations. XRF is widely used for lead paint and lead-in-bulk-material screening but is orthogonal to this invention's problem domain.

No widely available consumer product currently delivers laboratory-grade ppb sensitivity with visual in-home readout and minutes-to-hours time-to-result. This invention addresses that gap.

3. Problem Statement

The EPA drinking-water action level for lead is 10 ppb (15 ppb in the older Lead and Copper Rule; 10 ppb under the 2024 revisions). The American Academy of Pediatrics has recommended a drinking-water target of 1 ppb at school taps. Blood-lead evidence from decades of epidemiology (including NHANES and the Flint cohort) indicates that there is no safe level of lead exposure, and that chronic low-level dietary-water exposure is a meaningful contributor to total daily lead intake relative to the FDA Interim Reference Level (2.2 µg/day child, 8.8 µg/day adult).

Despite the clear public-health demand, consumer lead-in-water testing is stuck in a false dichotomy: cheap strips that miss the action level by a factor of 2–3, or mail-in lab kits that cost as much as and take longer than a municipal water report. The required tool, laboratory-grade sensitivity, fifteen-minute turnaround, $10-class cost, one-time-use cartridge, binary visual output, does not exist.

The invention's design goals are therefore:

• LoD ≤ 1 ppb dissolved Pb²⁺ in drinking water matrices (including hard water, chlorinated water, and water containing typical Cu²⁺/Zn²⁺/Fe interferents).
• Time-to-result ≤ 20 minutes, cartridge-to-read.
• No instrumentation beyond a consumer-grade 365 nm UV flashlight.
• Binary or semi-quantitative visual output, glow above a printed reference threshold indicates Pb > limit.
• Selectivity, the assay should not give a bright-green glow for Cu²⁺, Zn²⁺, Cd²⁺, Fe²⁺/³⁺, Ca²⁺, or Mg²⁺.
• Unit cost ≤ $10 at volume.

4. Theory of Operation

4.1 HSAB chelation at the thiol interface

The grafted silica surface presents pendant –(CH₂)₃–SH groups. At the pH of drinking water (typically 6.5–8.5), the thiol is partially deprotonated (pKa ≈ 10–11 for aliphatic thiols, depressed on silica to ≈ 8–9 by the neighbouring silanol environment), so both free –SH and anionic –S⁻ populations are present. Pb²⁺, a soft Lewis acid (Pearson η ≈ 6.8 eV, χ ≈ 3.9 eV), pairs strongly with the soft thiolate base. The Pb(S-R)₂ surface complex forms with apparent log K ≈ 16–17; competing drinking-water ions Ca²⁺ and Mg²⁺, which are hard, bind 10–12 orders of magnitude more weakly. Cu²⁺ binds well (log K ≈ 14–15) but does not interfere in the fluorescence step (see §6 below). The net effect of the thiol surface chemistry is a highly Pb-selective preconcentration filter.

A typical MPTS loading of 1.5 mmol SH per gram on 400 m²/g mesoporous silica, with a bed mass of 0.3 g, provides ≈ 4.5 × 10⁻⁴ mol of thiol. At full two-thiol coordination per Pb²⁺, the bed can capture ≈ 2.25 × 10⁻⁴ mol ≈ 47 mg of Pb before breakthrough, vastly in excess of the ≈ 1 µg target mass from a 1 L sample at 1 ppb. Capture is therefore capacity-unlimited for the consumer use case; the rate-limiting factor is mass transfer and residence time.

4.2 Tesla valvular conduit: residence time and recirculation diodicity

At the low Reynolds numbers typical of gravity-fed or peristaltic tap flow (Re ≈ 50–500 inside a 3 mm conduit at 0.1–1 mL/s), a Tesla valvular conduit exhibits diodicity, reverse-flow pressure drop exceeds forward-flow pressure drop by a factor of 1.5–2.2 (Thompson et al., verify). Two distinct Tesla-assisted flow architectures are contemplated:

Upstream residence-time enhancer (Embodiment B). A Tesla conduit placed immediately upstream of the sorbent bed induces vortex shedding and chaotic advection. The effective path length at a given bulk flow rate is extended 1.3–1.8×, increasing the probability of Pb²⁺ encountering a thiol site before exiting.

Captive recirculation loop (Embodiment A). A closed loop of sample reservoir → peristaltic pump → Tesla conduit → MPTS column → back to reservoir is run for N minutes. The Tesla element biases flow such that forward pumping encounters low resistance while any back-pressure transient encounters higher resistance, stabilizing one-directional recirculation without a mechanical check valve. For a 500 mL reservoir at 20 mL/min, the sample volume passes the bed 20 times in 10 minutes, driving apparent capture efficiency asymptotically to unity even for initial Pb²⁺ concentrations below the single-pass equilibrium limit.

4.3 In-situ MAPbBr₃ formation on captured Pb²⁺ sites

After the capture cycle, the bed is removed (or exposed in place, depending on embodiment) and sprayed uniformly with a solution of MABr (methylammonium bromide, CH₃NH₃Br) dissolved in isopropanol (typical concentration 50–200 mg/mL). Upon contact with a surface-bound Pb²⁺ site, the following stoichiometry applies:

Pb²⁺(surface) + 2 MABr + additional Br⁻ source → MAPbBr₃(s) + MA⁺

Additional bromide can be supplied by a slight excess of MABr in the spray (the MA⁺ counter-ion is released into the solvent phase), or by inclusion of a soluble bromide co-salt. Crystallization proceeds rapidly, nuclei appear on the timescale of seconds at room temperature from alcoholic solvent (Pellet et al., verify). Because Pb²⁺ is anchored to the thiol via a surface-bound Pb–S linkage, the MAPbBr₃ phase grows locally at each capture site rather than in bulk solution, which preserves spatial information about capture density and produces a bright, distinctly localized fluorescence signature on the bed.

4.4 Fluorescence emission and the Pb-selectivity of the 520 nm band

Under 365 nm excitation, MAPbBr₃ emits at ≈ 520 nm (green) with a photoluminescence quantum yield that is strongly morphology-dependent but is typically 10–50 % for the nanocrystalline phase grown from alcoholic precursor [Pellet et al.; Helmbrecht et al., verify]. This emission is the diagnostic signal. The 520 nm band is specific to the 3-D MAPbBr₃ perovskite: Cu²⁺, Zn²⁺, Cd²⁺, and Fe²⁺/³⁺ do not form the analogous 3-D perovskite at room temperature in alcoholic MABr, and therefore do not produce the bright-green emission. Cd²⁺ can form MACdBr₃ analogues under forcing conditions but not in the ambient MABr-in-isopropanol spray; in any case the emission would be in a different spectral band. Copper bromide phases (e.g., Cu–MA–Br) are non-emissive or weakly red-emissive. The upshot is that the combination of the thiolate capture step (which pre-selects for soft metals) and the MAPbBr₃ formation step (which produces green emission only for Pb²⁺) gives an orthogonal two-stage selectivity that rejects both hard-cation interferents (Ca, Mg, Fe) and the soft-cation near-neighbours (Cu, Zn, Cd) that would otherwise trouble a single-stage assay.

5. Embodiments

5.1 Embodiment A, Captive recirculation unit (countertop)

Physical layout. Sample reservoir (500 mL plastic cup or integrated tank) → silicone tubing → peristaltic pump (5 V DC, 10–50 mL/min) → Tesla valvular conduit (3 mm channel, five-stage, machined or injection-molded in PETG or polypropylene) → cartridge housing containing 0.3–1.0 g of MPTS-functionalized silica (60–100 µm particle size, bed volume ≈ 0.6–2.0 mL, polypropylene frit endcaps) → return line to reservoir.

Operational cycle. User fills reservoir with 500 mL of the water to be tested. Presses START. Pump runs for a pre-set duration (typically 10 minutes for drinking-water screening, 20 minutes for very-low-level screening). Pump stops.

Readout. Cartridge is unscrewed. Included trigger-spray bottle of MABr-in-isopropanol (100 mg/mL, ≈ 3 mL spray delivers ≈ 0.3 mL onto the bed) is applied uniformly over the exposed bed face. After 30–60 seconds for crystallization, the user illuminates the bed with a 365 nm LED flashlight (included; typical consumer unit, 3 W, filtered). Green glow at or above the printed reference intensity corresponds to Pb > threshold.

Fabrication protocol for MPTS silica. Silica gel (200–400 m²/g, 60–100 µm), dried at 120 °C overnight, refluxed 8 h in anhydrous toluene with 10 % v/v MPTS and a catalytic trace of triethylamine. Filter, wash with toluene, then ethanol, then water. Titratable thiol loading by Ellman's assay: target 1.2–2.0 mmol SH per gram. Store under argon.

5.2 Embodiment B, Throughput spigot cartridge (faucet-mounted)

Physical layout. A small injection-molded housing with standard faucet threads (15/16"-27 or 55/64"-27, with adapter). Inside: an inlet aerator-screen, a Tesla valvular conduit (acting as residence-time enhancer; five to seven stages in a spiraled 2 mm channel), the MPTS sorbent bed (0.2–0.4 g), an outlet flow restrictor to hold flow rate at ≈ 0.5–1.0 L/min regardless of household line pressure.

Operational cycle. User screws cartridge onto faucet. Opens cold tap. Runs for a defined cycle, typically 10 minutes, which at the restricted flow gives a 5–10 L processed volume. Closes tap. Unscrews cartridge.

Readout. Cartridge is opened at a marked seam (snap-fit), exposing the bed face. MABr spray from included trigger bottle. UV flashlight. Same read protocol as Embodiment A.

Consumables model. Cartridge + MABr spray bottle + reference card are single-use. UV flashlight is a reusable household item shipped with the starter kit.

5.3 Common materials and tolerances

• Silica support. Mesoporous silica gel (Davisil 646 grade or equivalent), 60–100 µm, 300 m²/g nominal, pore volume 1.1 mL/g. Alternatives: MCM-41 (higher surface area, higher cost), controlled-pore glass (narrower pore distribution).
• Silanization reagent. MPTS (Sigma-Aldrich 175617 or equivalent), handled under inert atmosphere because of the reactive methoxy groups.
• MABr. Commercial methylammonium bromide, ≥ 99 % purity, dried 24 h at 50 °C under vacuum, stored dry.
• Carrier solvent. Anhydrous isopropanol, spectroscopic grade. Isopropanol is selected over DMF/GBL on toxicity and consumer-safety grounds despite slightly slower crystallization.
• UV source. 365 nm LED, optical power ≥ 200 mW at the bed face (≈ 50 mW/cm² irradiance), visible-light cutoff filter (ZWB2 or equivalent) integrated in the consumer flashlight to eliminate purple leakage that would wash out green emission.

6. Expected Performance

6.1 Sensitivity

Using the capacity calculation of §4.1, and assuming capture efficiency ≥ 0.9 in the recirculation-loop embodiment, a 500 mL sample at 1 ppb Pb²⁺ contains 500 ng Pb total and deposits ≈ 450 ng Pb onto the bed. On a 0.5 cm² exposed bed face, this is ≈ 900 ng/cm² of anchored Pb. Conversion to MAPbBr₃ yields ≈ 2.6 µg/cm² of perovskite, well above the literature-reported visual-detection threshold for green emission under 365 nm illumination (tens of ng/cm² for unaided-eye detection in a darkened room; see Helmbrecht et al., verify). The invention is therefore theoretically capable of sub-ppb visual detection; a target LoD of 1 ppb is conservative.

6.2 Time-to-result

• Capture cycle: 5–15 min (configurable).
• MABr spray + crystallization: 30–90 s.
• UV readout: immediate.
• Total: 10–20 min cartridge-to-read.

6.3 Selectivity vs. common interferents

6.4 Stability and shelf life

• Dry MPTS silica is stable ≥ 24 months at ambient conditions in an oxygen-barrier sachet. Air-oxidation to disulfides can halve the free-thiol count over two years; a mild reducing-agent flush (dithiothreitol) restores activity but is not consumer-accessible, so the product is disposable.
• MABr-in-isopropanol is stable ≥ 12 months in an amber glass or coated-plastic bottle; water ingress will cause slow hydrolysis to methylamine + HBr and is the primary shelf-life limiter. Recommended packaging: barrier foil pouch for the trigger bottle.
• Assembled cartridge shelf life: 12 months, controlled-room-temperature.

7. Comparison Table

8. DRAFT CLAIMS, FOR ATTORNEY REVIEW

The following are inventor-drafted claims for attorney polishing. Scope and dependent-claim strategy to be determined after prior-art search.

Independent claims

Claim 1. A field-deployable apparatus for detecting lead in aqueous solution, comprising: (a) a sorbent bed comprising thiol-functionalized silica particles having pendant mercapto groups configured to chelate Pb²⁺ from a flowing aqueous sample; (b) a fluidic conduit upstream of the sorbent bed, said fluidic conduit comprising at least one asymmetric-flow-resistance structure configured to increase the residence time of the aqueous sample within the sorbent bed or to bias a closed recirculation loop in a forward direction without a moving check valve; (c) a supply of methylammonium bromide in an alcoholic carrier, configured for post-capture application to the sorbent bed such that captured Pb²⁺ reacts in situ with the methylammonium bromide to form methylammonium lead bromide perovskite crystals at each capture site; and (d) an ultraviolet excitation source of wavelength between 350 nm and 395 nm configured for illumination of the sorbent bed after said application, whereby photoluminescence at approximately 520 nm from said perovskite crystals provides a visual indication of captured lead mass.

Claim 2. A method for detecting lead in potable water, comprising the steps of: (a) flowing a water sample through a bed of mercapto-functionalized silica so as to capture Pb²⁺ from the sample onto the bed by soft-acid/soft-base thiolate chelation; (b) during said flowing step, routing the sample through an asymmetric-flow-resistance conduit such that the aqueous sample passes the bed either once with enhanced residence time or a plurality of times in a closed recirculation loop; (c) applying to the bed, after said flowing step, a solution of methylammonium bromide in a polar alcoholic solvent, such that the methylammonium bromide reacts with captured Pb²⁺ in situ on the bed to form methylammonium lead bromide perovskite crystals at lead-binding sites; and (d) exposing the bed to ultraviolet light between 350 nm and 395 nm and visually evaluating the resulting green photoluminescence as an indication of the lead content of the original water sample.

Claim 3. A consumable cartridge configured to screw onto a residential faucet, comprising an inlet adapted to household thread standards, a first internal volume containing an asymmetric-flow-resistance conduit of the Tesla valvular type, a second internal volume containing mercapto-functionalized silica, and an outlet flow restrictor configured to maintain flow rate within a range suitable for the capture chemistry of claim 1, wherein the cartridge is further adapted to be opened after use to expose the sorbent bed to a post-capture reagent spray.

Dependent claims

Claim 4. The apparatus of claim 1 wherein the thiol-functionalized silica comprises mesoporous silica gel with a surface area between 200 m²/g and 600 m²/g functionalized with (3-mercaptopropyl)trimethoxysilane at a thiol loading between 0.8 mmol/g and 2.5 mmol/g.

Claim 5. The apparatus of claim 1 wherein the asymmetric-flow-resistance structure is a Tesla valvular conduit having between three and ten cascaded stages and a diodicity ratio of at least 1.3 in the operating Reynolds-number range.

Claim 6. The apparatus of claim 1 further comprising a peristaltic pump and a sample reservoir arranged with the sorbent bed and the asymmetric-flow-resistance structure in a closed recirculation loop.

Claim 7. The method of claim 2 wherein step (b) comprises passing a fixed sample volume through the bed a plurality of N times, where N is at least 5, via said closed recirculation loop.

Claim 8. The method of claim 2 wherein the alcoholic solvent of step (c) is isopropanol and the concentration of methylammonium bromide is between 50 mg/mL and 250 mg/mL.

Claim 9. The method of claim 2 wherein the ultraviolet light source of step (d) is a light-emitting diode with peak emission at 365 ± 5 nm and an integrated visible-light cutoff filter.

Claim 10. The cartridge of claim 3 wherein the cartridge is single-use and provided in a kit together with a spray bottle of methylammonium bromide in isopropanol and a 365 nm ultraviolet flashlight.

Claim 11. The apparatus of claim 1 wherein the green photoluminescence at approximately 520 nm is selective for Pb²⁺ over Cu²⁺, Zn²⁺, Cd²⁺, Fe²⁺, Fe³⁺, Ca²⁺, and Mg²⁺ at drinking-water-relevant concentrations.

Claim 12. The method of claim 2 further comprising the step of comparing the observed green photoluminescence to a printed reference intensity card to produce a semi-quantitative estimate of dissolved lead concentration in the original water sample, binned into ranges including at least one bin corresponding to concentrations below 1 part per billion and at least one bin corresponding to concentrations above 10 parts per billion.

Claim 13. The cartridge of claim 3 wherein the mercapto-functionalized silica is provided as a bed of mass between 0.1 g and 1.0 g contained between two polypropylene frits.

9. Figure Captions

Figures to be rendered separately.

Figure 1. System schematic, Embodiment A (captive recirculation). Block diagram showing sample reservoir, peristaltic pump, Tesla valvular conduit, MPTS sorbent cartridge, and return line, with annotated flow direction, typical flow rate (20 mL/min), and cycle duration (10 min).

Figure 2. System schematic, Embodiment B (spigot cartridge). Cross-section of the faucet-mounted cartridge showing faucet thread, inlet screen, Tesla conduit (spiraled 5-stage), MPTS bed, outlet flow restrictor, and snap-fit access seam for post-capture bed exposure.

Figure 3. Molecular schematic, MPTS-silica surface chemistry. Representation of silica support with grafted (3-mercaptopropyl)silane monolayer, free –SH and partially deprotonated –S⁻ at drinking-water pH, and Pb(SR)₂ surface complex. Inset shows HSAB classification.

Figure 4. Tesla valvular conduit geometry. Plan view of the five-stage cascade used in this invention, with key dimensions (channel width 3 mm, stage pitch 25 mm, overall length 150 mm) and forward/reverse flow arrows. Adapted from with modifications for injection-molding.

Figure 5. Diodicity plot. Expected forward and reverse pressure drop vs. Reynolds number for the Figure-4 geometry, with diodicity ratio Δp_reverse / Δp_forward on secondary axis, Re range 50–500 covering gravity-fed and peristaltic-pump flows.

Figure 6. In-situ MAPbBr₃ crystallization mechanism. Reaction schematic: captured Pb–S–silica + 2 MABr (in isopropanol) → MAPbBr₃(s) + 2 MA⁺ + 2 HS–silica (disrupted thiolate) or alternatively → MAPbBr₃(s) nucleated at the site with thiolate liberated by ligand exchange. Both pathways shown.

Figure 7. Photoluminescence spectrum of MAPbBr₃. Representative normalized emission spectrum under 365 nm excitation, λ_em ≈ 520 nm, FWHM ≈ 20 nm. Reproduced / adapted from [Pellet et al., Nature, 2018, verify].

Figure 8. Selectivity panel. Photograph grid (to be taken) showing bed appearance under 365 nm illumination after exposure to identical-concentration solutions of Pb²⁺, Cu²⁺, Zn²⁺, Cd²⁺, Fe³⁺, and a blank. Bright green only for Pb²⁺.

Figure 9. Calibration curve. Visual-detection-threshold curve: plotted perovskite fluorescence intensity (arbitrary units, camera-quantified) vs. original-sample Pb²⁺ concentration (0.1–100 ppb, log axis), with recirculation cycle time as a parameter (5, 10, 20 min). Shaded region indicates unaided-eye visual threshold.

Figure 10. Consumer-facing read card. Printed reference card with three green swatches corresponding to < 1 ppb, 1–10 ppb, and > 10 ppb, for direct user comparison against the illuminated bed.

10. References

Real references only. Citations flagged ", verify" require bibliographic detail confirmation before filing.

1. Tesla, N. Valvular Conduit. 1920.
2. Pellet, N. et al. [MAPbBr₃ perovskite fluorescence / Pb²⁺ sensing paper, Nature, 2018, verify full citation].
3. Helmbrecht, L. et al. [MABr paper indicator for Pb²⁺ on surfaces, Environmental Science & Technology, 2023, verify full citation, DOI, volume, page].
4. Pearson, R. G. Hard and soft acids and bases. Journal of the American Chemical Society 85(22), 3533–3539 (1963). [Canonical HSAB reference, verify precise page if cited in final filing.]
5. Feng, X. et al. Functionalized monolayers on ordered mesoporous supports. Science 276, 923–926 (1997). [Representative MPTS/mercapto-silica sorbent paper, verify.]
6. Mercier, L. and Pinnavaia, T. J. Access in mesoporous materials: advantages of a uniform pore structure in the design of a heavy-metal ion adsorbent for environmental remediation. Advanced Materials 9(6), 500–503 (1997). [Representative mesoporous thiol sorbent, verify.]
7. Jal, P. K., Patel, S., and Mishra, B. K. Chemical modification of silica surface by immobilization of functional groups for extractive concentration of metal ions. Talanta 62(5), 1005–1028 (2004). [Review of thiol-silica SPE, verify.]
8. Thompson, S. M. et al. Numerical investigation of multistaged Tesla valves. Journal of Fluids Engineering (ASME), verify year, volume, pages.
9. Nguyen, Q. M. et al. Tesla valves at low Reynolds number. Micromachines, verify year, volume, pages.
10. U.S. Environmental Protection Agency. Lead and Copper Rule. 40 CFR Part 141, Subpart I. [Revised 2024, verify current CFR citation.]
11. U.S. Food and Drug Administration. Interim Reference Levels for lead in food. [FDA guidance document, verify current citation; 2.2 µg/day children / 8.8 µg/day adults.]
12. American Academy of Pediatrics. Prevention of childhood lead toxicity. Pediatrics, verify year/volume/pages; 1 ppb school-tap target.

End of disclosure v1. Length approximately 4,400 words. Next step: attorney prior-art search per companion checklist.