Project Edible electronics

Lab Handout: Curcumin–Chitosan Thin Films (Air Oven + Silver Paste)

LAB HANDOUT (SOP): Curcumin–Chitosan Thin Films

For UV–Vis pH response, FTIR confirmation, electrical readout (graphite/silver electrodes), and optional NLO Z-scan (salt-free samples).

Project: Biopolymer indicator film with dual optical + electrical readout

Film System: Chitosan + Curcumin (± NaCl)

Drying: Air oven (two-stage) for smooth films

Electrodes: Graphite (edible-inspired) + Silver paste (benchmark contact)

1) Objective

  • Prepare uniform thin films of chitosan–curcumin with reproducible thickness and color.
  • Produce salt-free films for optical/NLO tests and salt-added films for improved electrical response.

2) Safety & Rules (Mandatory)

  1. Wear gloves + lab coat + goggles.
  2. Work with glacial acetic acid carefully; prepare dilute acid in a ventilated area.
  3. Keep films away from direct sunlight during drying (curcumin photobleaches).
  4. Use the term “biopolymer indicator film” in notes; do not claim “edible” without dose justification.

3) Materials Required

Item Purpose Notes
Chitosan Film-forming polymer Soluble in dilute acetic acid
Curcumin pH indicator dye Use low loading first to avoid aggregation
Glacial acetic acid + DI water Prepare 1% acetic acid for chitosan dissolution Measure accurately
NaCl (optional) Enhance ionic conduction for electrical sensing Do not use for Z-scan films (may increase scattering)
Graphite Electrode pads (edible-inspired) Good for quick tests; contact may vary
Silver paste Benchmark stable electrodes Not edible; best for repeatable electrical data
Glass slides / petri dishes Film casting substrates Clean thoroughly for smooth films

4) Sample Plan (Minimum)

Sample ID Composition Use
C0 Chitosan only Control
C1 Chitosan + 0.5 wt% curcumin UV–Vis, FTIR, NLO (preferred), baseline electrical
C2 Chitosan + 1.0 wt% curcumin Higher optical response (may scatter more)
C1S C1 + NaCl (ionic) Electrical sensing (do not use for Z-scan)

5) Synthesis Procedure (Quantitative)

Step 1 — Prepare 1% Acetic Acid (100 mL)

  1. Add 99 mL DI water to a clean bottle/beaker.
  2. Add 1.00 mL glacial acetic acid.
  3. Mix well and label: “1% Acetic Acid”.

Step 2 — Prepare 1.5 wt% Chitosan Stock (50 mL)

  1. Pour 50 mL of 1% acetic acid into a beaker.
  2. Weigh 0.750 g chitosan (electronic balance).
  3. Sprinkle slowly into the beaker while stirring.
  4. Stir for 4–6 hours (or overnight) until uniform viscous solution forms.
  5. De-bubble: keep undisturbed for 30 minutes.
  6. Label: “CS Stock (1.5%)”.

Step 3 — Curcumin Loading Calculations (for 15 mL portions)

Chitosan in 15 mL portion:

mCS(15 mL) = 0.750 × (15/50) = 0.225 g

C1 (0.5 wt%): mcur = 0.005 × 0.225 g = 0.001125 g = 1.125 mg

C2 (1.0 wt%): mcur = 0.01 × 0.225 g = 0.00225 g = 2.25 mg

Step 4 — Prepare Film Solutions

  1. Divide CS stock into three labeled beakers: 15 mL + 15 mL + 15 mL (C0, C1, C2).
  2. C0: add nothing.
  3. C1: add 1.13 mg curcumin; stir 30 min.
  4. C2: add 2.25 mg curcumin; stir 30 min.
Tip: If curcumin aggregates, extend stirring time. Keep dye loading low for best optical clarity.

Optional Step — Prepare Salt Sample for Electrical Readout (C1S)

  1. Take 15 mL of C1 into a new beaker labeled C1S.
  2. Add 0.050 g (50 mg) NaCl and stir 15 min until dissolved.
Important: Do not use NaCl-added films for Z-scan/NLO; salt can increase scattering and water uptake.

6) Substrate Cleaning (Do Not Skip)

  1. Rinse slides with DI water.
  2. Rinse with acetone and wipe clean.
  3. Dry fully; do not touch casting area with fingers.

7) Film Casting (Uniform Thickness)

  1. Level the casting surface.
  2. Dispense 5.00 mL solution onto each slide (same volume for all samples).
  3. Spread gently with a glass rod in one direction; avoid bubbles.
  4. Leave for 10 minutes at room temperature for leveling.

8) Air-Oven Drying (Improves Film Quality)

  1. Stage 1: 35–40 °C for 1 hour (slow drying → fewer cracks).
  2. Stage 2: 50 °C for 1–2 hours (complete drying).
  3. Cool inside oven (door slightly ajar) to room temperature.
Do not exceed 60 °C (curcumin stability + cracking risk). If cracking occurs: reduce temperature and increase time.

9) Film Peeling & Conditioning

  1. Peel carefully after full cooling (or keep on slide for optical tests).
  2. Condition in desiccator/dry box overnight.
  3. Record mass, appearance, cracks (yes/no), thickness (if available).

10) Electrode Preparation (Graphite + Silver Paste)

Option A — Graphite Electrodes (Edible-inspired)

  1. Mark two pads (e.g., 10 mm × 10 mm) separated by a 3–5 mm gap.
  2. Rub graphite uniformly to form conductive pads.
  3. During measurement, place probes at the same spot each time (consistent pressure).

Option B — Silver Paste Electrodes (Benchmark, Best Repeatability)

  1. Mark the same pad geometry (10 mm × 10 mm; gap 3–5 mm).
  2. Apply thin silver paste pads with a toothpick.
  3. Cure in air oven at 50–60 °C for 10–15 minutes (or per paste instructions).
  4. Attach wires if needed; avoid shorting across the gap.
Note: Silver paste is not edible. Use it as a benchmark for electrical measurements and compare with graphite electrodes.

11) Z-Scan / NLO Sample Selection (Reminder)

  • Use C1 (0.5% curcumin) without NaCl for best optical clarity.
  • Keep electrode area away from laser path; use separate pieces for NLO tests.

12) Deliverables (Student Checklist)

  • Weighing/calculation sheet (all masses)
  • Preparation log (times, stirring conditions)
  • Drying program (temperature + time)
  • Photos of films (wet cast + dried)
  • Film mass + notes on cracks/bubbles
  • Electrode geometry (pad size + gap)
Lab Note: Keep one set of films strictly salt-free (optical/NLO). Keep a separate set with NaCl for electrical sensing. Store all films in the dark in a dry container.

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