DiscSPR

I — Abstract

Low-cost SPR sensing via optical disc gratings

More than 2 billion people lack access to safe drinking water. Current water monitoring methods require centralized labs and costly equipment, precluding routine monitoring in resource-poor environments. This research presents DiscSPR: a grating-coupled SPR (GC-SPR) platform using commercial BD-R, DVD-R, and CD-R optical discs — which already contain nanoscale grooves — as inexpensive substitutes for traditional plasmonic gratings. Discs were coated with 40–70 nm Ag or Cu by magnetron sputtering, assembled into 3D-printed microfluidic flow cells, and characterized by reflectance wavelength modulation validated against RCWA simulations. Ag-coated BD-R provided the sharpest resonances (321 nm/RIU); CD-R achieved the highest sensitivity (394 nm/RIU). Sensor material cost: under $10 per unit, vs. $50,000–$150,000 for commercial GC-SPR.

CoverOptical disc repurposed as a GC-SPR sensor chip.

1 — Introduction

The water monitoring gap

Over 2 billion people lack safe water. Contaminated water causes 485,000 deaths annually from diarrhea alone. The key barrier is not science — it is cost and accessibility of detection tools.

The three main classes of waterborne toxins are: heavy metals (Pb, Hg) causing irreversible organ damage and developmental disorders; pesticides (e.g., atrazine) disrupting hormones and raising cancer risk; and microbial toxins (cyanotoxins, endotoxins) causing acute illness. Contaminated water is colorless and odorless — undetectable without testing.

Modern testing (ICP-MS, HPLC) requires specialized labs costing $20,000–$200,000. This gap defines the engineering challenge DiscSPR addresses.

Water contaminant explorer

Select a contaminant class to compare levels and WHO limits

Typical affected region level

WHO limit

DiscSPR detection target

Select a category above to explore contamination data.

Global water testing access map

Hover a country for access, cost, and key contamination concern.

High

Moderate

Poor

None

—

Access—

Cost—

Key concern—

Interactive map — world data loads from CDN. Hover countries for water testing access and contamination data.

Why GC-SPR? Why optical discs?

SPR is a gold-standard optical biosensing technique — label-free, real-time, femtomolar sensitivity. The traditional Kretschmann prism setup costs $50,000–$200,000 and is non-portable. Grating-coupled SPR (GC-SPR) replaces the prism with a periodic grating, enabling compact, miniaturizable sensors — but conventional GC-SPR still requires expensive nanofabrication (e-beam lithography, FIB milling).

DiscSPR's insight: commercial optical discs are already mass-manufactured precision nanoscale gratings — at less than one cent per item — with grating periods of 322 nm (BD-R), 805 nm (DVD-R), and 1,582 nm (CD-R). No nanofabrication needed.

Deployment cost calculator

Compare DiscSPR vs. traditional GC-SPR for field deployment

Number of sensors

50 sensors

Field sites

10 sites

Sensors replaced per year

20%/year

DiscSPR total cost

$21,500

Apparatus + sensors + annual replacement

vs.

Traditional GC-SPR total cost

$1,100,000

$100k/apparatus + $200/chip replacement

Savings

$1,078,500

98.0% cost reduction

Figure 2Grating-coupled SPR (GC-SPR) — prism replaced by a periodic grating on the metal film for compact, miniaturizable sensing.

Figure 3DiscSPR concept: optical disc replaces custom-fabricated grating. Polarized light excites SPP wave at metal/dielectric interface; spectrometer detects resonance dip shift.

Figure 4Groove structure comparison across disc formats. Grating periods: BD-R 322 nm, DVD-R 805 nm, CD-R 1,582 nm.

2 — Materials & Methods

From optical disc to plasmonic sensor

1

Step 01

Select & cut

BD-R, DVD-R, CD-R cut to 2.5 × 2.5 cm². Only recordable (-R) media — permanent grating embossing ensures consistency.

2

Step 02

Strip & clean

Metal/dye layers dissolved in 70% nitric acid (15 min BD-R; 5 min DVD/CD-R). Rinsed by IPA and ultrasonic bath.

3

Step 03

Sputter coat

Ag or Cu (99.99%) deposited by magnetron sputtering at 40, 50, or 70 nm nominal thickness (Denton Vacuum Desktop Pro).

4

Step 04

Measure

Coated disc in 3D-printed PLA flow cell. Broadband p-pol QTH light; AvaSpec spectrometer records Ip/Is at 30°–70°.

Figure 5Disc cutting process: discs sectioned to 2.5 × 2.5 cm² squares.

Figure 6Cleaning sequence: before 70% nitric acid → sonication rounds → final cleaned substrate.

Figure 7Custom 3D-printed SPR microfluidic device (Bambu Lab P1S, PLA). CAD model, cross-section, and front/back photographs with inlet/outlet channels labeled.

AFM characterization of BD-R DVD-R and CD-R

Figure 8AFM surface topography and grating parameters (5 µm × 5 µm scans, n = 3 positions). BD-R: Λ = 322 ± 5.2 nm, h_g = 23.9 ± 0.7 nm. DVD-R: Λ = 805 ± 7.5 nm, h_g = 158.6 ± 2.2 nm. CD-R: Λ = 1582 ± 13 nm, h_g = 151.8 ± 2.9 nm.

Disc	Grating period (Λ)	Groove depth (hg)	Metal continuity
BD-R	322 ± 5.2 nm	23.9 ± 0.7 nm	Continuous (bridges grooves)
DVD-R	805 ± 7.5 nm	158.6 ± 2.2 nm	Less continuous
CD-R	1582 ± 13 nm	151.8 ± 2.9 nm	Less continuous

Disc substrates before and after Cu or Ag deposition

Figure 9Optical disc grating substrates before sputtering and after deposition of thin copper (Cu) or silver (Ag) films for all three disc types.

Figure 10Home-built WMS setup: QTH lamp, collimating/polarizing optics, motorized θ–2θ stage, AvaSpec fiber-coupled spectrometer. Total apparatus cost ~$200.

3 — Results

Experimental results

3.1 — Sensitivity

Sensitivity results for sensors with ethylene glycol–water solutions

Ag-coated optical disc sensors were evaluated using ethylene glycol–water mixtures of increasing refractive index. CD-R showed the largest wavelength shift per refractive index unit, while BD-R produced the sharpest and most reproducible resonance dips.

Refractive index sensitivity — Ag-coated discs

Front-side illumination · 50° incidence · ethylene glycol/water (0–100% w/w) · n = 3

CD-R

394 nm/RIU

R² = 0.990 · highest sensitivity

BD-R

321 nm/RIU

R² = 0.933 · sharpest dips

DVD-R

290 nm/RIU

R² = 0.998 · best linearity

Disc	S (nm/RIU)	R²	Resonance quality	Best trait
CD-R	394	0.990	Broad, overlapping modes	Highest sensitivity
BD-R	321	0.933	Sharp, well-defined	Best resonance quality
DVD-R	290	0.998	Moderate definition	Best linearity

Figure 11 Illumination geometry for grating-coupled SPR using metal-coated optical discs. In (a) front-side mode, light couples directly at the metal–water interface; in (b) back-side mode, light first traverses the ~1 mm polycarbonate substrate, introducing phase distortions. Key grating parameters: period (Λ), groove depth (h_g), metal film thickness (h_m). Front-side illumination is preferred for all three disc substrates.

3.2 — Front-side vs. back-side, Ag vs. Cu

SPR results: front-side vs. back-side illumination, silver vs. copper, RCWA vs. experimental

Front-side illumination produced cleaner, sharper, and more interpretable resonances across all substrates, while back-side illumination produced more complex spectral features due to transmission through the polycarbonate substrate. To make this comparison immediately clear, the front-side data are shown first, followed by the back-side data.

      FRONT-SIDE ILLUMINATION
    

Silver (Ag) FRONT-SIDE · Experimental vs. RCWA

Copper (Cu) FRONT-SIDE · Experimental vs. RCWA

Front-side illumination — Silver (Ag) coated BD-R, DVD-R, CD-R

Figure 12a — Front-side · Silver (Ag) Front-side illumination: Ag-coated BD-R (a,b), DVD-R (c,d), and CD-R (e,f) — experimental spectra (left column) vs. RCWA simulations (right column) at 40, 50, and 70 nm film thicknesses.

Front-side illumination — Copper (Cu) coated BD-R, DVD-R, CD-R

Figure 12b — Front-side · Copper (Cu) Front-side illumination: Cu-coated BD-R (a,b), DVD-R (c,d), and CD-R (e,f) — experimental spectra (left column) vs. RCWA simulations (right column) at 40, 50, and 70 nm film thicknesses.

Figure 12 Front-side illumination comparison for Silver (left) and Copper (right) across BD-R, DVD-R, and CD-R — experimental data vs. RCWA simulated data at 40, 50, and 70 nm film thicknesses. This is the preferred illumination geometry because light couples directly at the metal–water interface, giving cleaner and more interpretable resonance spectra.

      BACK-SIDE ILLUMINATION
    

Silver (Ag) BACK-SIDE · Experimental vs. RCWA

Copper (Cu) BACK-SIDE · Experimental vs. RCWA

Back-side illumination — Silver (Ag) coated BD-R, DVD-R, CD-R

Figure 13a — Back-side · Silver (Ag) Back-side illumination: Ag-coated BD-R (a,b), DVD-R (c,d), and CD-R (e,f) — experimental spectra (left column) vs. RCWA simulations (right column) at 40, 50, and 70 nm film thicknesses.

Back-side illumination — Copper (Cu) coated BD-R, DVD-R, CD-R

Figure 13b — Back-side · Copper (Cu) Back-side illumination: Cu-coated BD-R (a,b), DVD-R (c,d), and CD-R (e,f) — experimental spectra (left column) vs. RCWA simulations (right column) at 40, 50, and 70 nm film thicknesses.

Figure 13 Back-side illumination comparison for Silver (left) and Copper (right) across BD-R, DVD-R, and CD-R — experimental data vs. RCWA simulated data at 40, 50, and 70 nm film thicknesses. In this geometry, light first passes through the ~1 mm polycarbonate substrate, which introduces additional phase perturbations and makes the spectra more complex than in front-side illumination.

3.3 — Dispersion

Surface plasmon dispersion plot results

Surface plasmon dispersion analysis confirms the plasmonic origin of the resonances. Experimental resonance positions follow the predicted surface plasmon polariton branches more closely under front-side illumination than under back-side illumination.

SP dispersion plots front-side illumination

Figure 14Surface plasmon dispersion plots for Ag- and Cu-coated optical discs under front-side illumination, showing agreement between experimental resonance positions and RCWA-predicted SPP branches.

SP dispersion plots back-side illumination

Figure 15Surface plasmon dispersion plots under back-side illumination. Larger deviations from predicted curves are consistent with substrate-induced phase perturbations.

4 — Discussion

What the results mean

Sensitivity vs. resonance sharpness

A fundamental tradeoff exists: CD-R's large period produces broader resonances with larger wavelength shifts per RI unit (394 nm/RIU), while BD-R produces sharper, more reproducible resonances at 321 nm/RIU. For analytical applications, front-side Ag-coated BD-R represents the optimal balance.

Front-side illumination preferred

Back-side illumination causes geometric interference through the 1 mm polycarbonate substrate, weakening the field at the metal-dielectric interface. Only BD-R showed reasonable back-side performance. Front-side illumination is recommended for all disc-based GC-SPR applications.

Silver vs. copper

Silver's lower intrinsic optical loss produces cleaner resonances. Copper oxidizes in air, forming Cu₂O/CuO surface layers. Protective coatings (graphene, MoS₂) could stabilize Cu films in future work.

Real-world impact

One DiscSPR sensor costs under $10; a complete system costs ~$2,000 vs. $50,000–$150,000 for commercial GC-SPR. The Flint, Michigan lead crisis — affecting over 100,000 people — illustrates exactly the scenario where affordable, deployable sensing saves lives.

Cost Metric	DiscSPR (this work)	Traditional GC-SPR
Grating substrate	<$1 (optical disc)	$100+ per chip
Materials per sensor unit	~$4–7	$200+
Complete sensing system	~$2,000	$50,000–$150,000

Limitations

Copper stability: Cu oxidizes at room temperature. Protective layers (graphene, MoS₂, Al₂O₃) will be investigated. Molecular recognition not yet proven: this study establishes the plasmonic platform; lead-specific aptamers must be incorporated to quantify LOD and selectivity. Portability: current setup is benchtop-sized; a smartphone-coupled version could reduce cost below $50.

5 — Future Work

Next steps toward field deployment

01

Surface functionalization

Aptamers, antibodies, or MIPs for selective detection of microcystin, atrazine, PFAS, and Pb²⁺.

02

Copper stabilization

Graphene or MoS₂ overlayers to prevent oxidation and extend Cu sensor lifetime.

03

Smartphone readout

Replace benchtop spectrometer with smartphone-based spectral sensor — apparatus cost below $50.

04

Prism-grating hybrid

Prism on substrate side to enhance back-side coupling for strongly absorbing or colored water samples.

05

Real water validation

Test in lake, groundwater, and tap water with known contaminant additions — establish practical LOD in complex matrices.

06

Multiplexed detection

SPR imaging with multiple functionalized zones on one disc — simultaneous multi-toxin detection in one measurement.

6 — Conclusions

Summary

This work demonstrates the first systematic study of optical discs as GC-SPR substrates, validated by RCWA simulations from AFM measurements. Key results: (1) optical discs support GC-SPR excitation without nanofabrication; (2) front-side Ag-coated BD-R gives sharpest resonances and 321 nm/RIU sensitivity with highest repeatability; (3) CD-R achieves highest sensitivity at 394 nm/RIU; (4) silver outperforms copper; (5) front-side illumination is preferred. Sensor material cost is under $10 per unit — over 99% cheaper than commercial GC-SPR. The problem is not in science but in availability and cost. DiscSPR demonstrates that optical disc SPR platforms can help solve this.