Polymer distributed Bragg reflectors: an old structure with unexpected sensing capabilities

Introduction Polymer distributed Bragg reflectors (DBRs) are gaining attention thanks to mass fabrication technologies, lightweight, and flexibility, which are unconceivable with other photonic structures.1 Indeed, they are... [ view full abstract ]

Introduction

Polymer distributed Bragg reflectors (DBRs) are gaining attention thanks to mass fabrication technologies, lightweight, and flexibility, which are unconceivable with other photonic structures.¹ Indeed, they are increasingly studied for lasing,² fluorescence enhancement,^3-5 and optical switching.⁶ Moreover, their easy assimilation with packaging fabrication technologies⁷ makes them interesting integrated transductors for barrier polymers employed in the encapsulation of moisture-sensitive devices. Notwithstanding the low permeability of barrier polymers, small molecules like water and oxygen can permeate thought the polymers affecting the device. We demonstrate that when the barrier films are structured to form a DBR, such intercalation allows to evaluate in-situ the molecular diffusion parameters, which are currently assessed ex-situ with complex gravimetric methods. Indeed, when a small molecule intercalates into a DBR the interaction between the polymer and the molecules allows a shift of the photonic band-gap with kinetics and magnitude that depends on the chemico-physical interactions between the two players.^8-9 Such effects, which are unconceivable with inorganic systems, consents to esteem diffusivity and allows simple colorimetric selective label-free transductors for a variety of pollutants.

Methods

DBRs were fabricated by spin-coating of alternated layer of polystyrene (PS) and cellulose acetate (CA) solutions. More details on preparation and optical characterization are reported in Ref.^8-9

Results

Figure 1a shows the reflectance of a PS:CA DBR before and after exposure to methanol vapors. Before the exposure the structure displays a maximum assigned to the stop-band at 840 nm (purple), which shift to ~940 nm after 90 min in methanol rich environment (red). From the dynamics of the stop-band shift, it is possible to retrieve the optical sorption-curves reported in Figure 1b for methanol and other alcohols. There, the stop-band reaches a plateau within 90 min for methanol, while heavier alcohols display longer kinetics.

Discussion

The data of Figure 1b deliver information on the analyte diffusivity within the polymers, which depends on free-volume, polarity, weak-bonding, and molecular size.^8-9 Then, we can retrieve, for instance, diffusivity values of ~10^-8 cm²/min for methanol, in full agreement with gravimetric data.^10-11 Moreover, the DBR response allows label-free selectivity, which nowadays requires long time sampling, separation processes and analyses.