Origins of Polymer Autofluorescence
Polymer autofluorescence arises from: (i) aromatic rings present in polystyrene, polycarbonate, and ABS — these emit broadly at 300–450 nm under 280–340 nm excitation; (ii) residual photoinitiators and processing additives in UV-cured adhesives and coatings — particularly Irgacure 819, Darocur 1173, and HALS stabilisers; (iii) oxidative chain-end chromophores (carbonyl groups at chain scission sites) formed during thermal processing; and (iv) trace biogenic contaminants (FADH₂, collagen) in bio-grade polymers. For fluorescence-based LFAs with reporters in the 510–670 nm range (excited at 450–640 nm), polymers with aromatic rings and photoinitiator residues are the most critical concern.
Material Screening Method & Results
Six polymer candidates were characterised using a Horiba FluoroMax-4 spectrofluorometer (2nm/nm bandpass, detector PMT, 0.5s integration) using front-face geometry on 1mm flat coupons. Excitation wavelengths tested: 450, 471, 532, and 635 nm (matching common fluorescent reporter excitation). Emission intensity measured at peak+30nm bandwidth. Results: PMMA (PLEXIGLAS V825T): 0.024 a.u. at 471nm ex; Polycarbonate (Makrolon): 0.019 a.u.; ABS: 0.041 a.u.; COC (TOPAS 6013): 0.007 a.u.; COP (ZEONEX 480R): 0.004 a.u.; COP + UV stabiliser (Tinuvin 326, 0.5%): 0.003 a.u.
Processing Parameter Effects on Autofluorescence
For COP (the preferred substrate), injection moulding melt temperature was the dominant autofluorescence driver: at 260°C melt temperature, autofluorescence at 471nm excitation = 0.003 a.u.; at 290°C, = 0.008 a.u. (2.7× increase). The increase corresponds to oxidative carbonyl formation confirmed by FTIR (C=O stretch at 1,740 cm⁻¹ increased by 340% at 290°C). Mould atmosphere: nitrogen purging reduces autofluorescence by 23% vs. air-exposed processing at 260°C. UV bleaching (302 nm, 400 mW/cm², 30 minutes) reduces PMMA autofluorescence from 0.024 to 0.009 a.u. — still 3× higher than COP, confirming that bleaching cannot substitute for material selection.
Impact on Assay Performance: CRP Detection Case Study
The practical impact was quantified using a lateral flow CRP assay (Alexa Fluor 532 reporter, anti-CRP capture line on 1mm substrate coupon). The reader excited at 532 nm, emission collected at 590 nm. On PMMA substrates: signal/background (S/B) ratio at 5 mg/L CRP = 4.1, LoD = 2.8 mg/L. On COP/UV-stabilised substrate: S/B = 21.4, LoD = 0.38 mg/L. The 7.4× reduction in autofluorescence translated to a 7.4× LoD improvement — confirming that substrate autofluorescence, not antibody affinity, was the assay-limiting factor on PMMA. COP substitution required no assay reformulation — only substrate change.