Flory-Huggins Photonic Sensors for the Optical Assessment of

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Flory-Huggins Photonic Sensors for the Optical Assessment of Molecular Diffusion Coefficients in Polymers Paola Lova, Giovanni Manfredi, Chiara Bastianini, Carlo Mennucci, Francesco Buatier de Mongeot, Alberto Servida, and Davide Comoretto ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b03946 • Publication Date (Web): 16 Apr 2019 Downloaded from http://pubs.acs.org on April 16, 2019

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ACS Applied Materials & Interfaces

Flory-Huggins Photonic Sensors for the Optical Assessment of Molecular Diffusion Coefficients in Polymers

Paola Lova,1* Giovanni Manfredi,1○ Chiara Bastianini,1 Carlo Mennucci,2 Francesco Buatier de Mongeot,2 Alberto Servida,1 and Davide Comoretto1*

1

Dipartimento di Chimica e Chimica Industriale, Università degli Studi di Genov, Via Dodecaneso 31, 16146, Genova, Italy

2 Dipartimento

di Fisica, Università degli Studi di Genova, Via Dodecaneso 31, 16146,

Genova, Italy

○Present Address: Center for Nano Science and Technology, Istituto Italiano di Tecnologia, Via Giovanni Pascoli 70, 20133 Milano, Italy

ACS Paragon Plus Environment

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KEYWORDS: Diffusion coefficient, distributed Bragg reflectors, Flory-Huggins, photonic crystals, volatile organic compond sensing.

Abstract: The lack of cost-effective systems for the assessment of air pollutants is a concern for health and safety in urban and industrial areas. The use of polymer thin-films as label-free colorimetric sensors featuring specific interactions with pollutants would then represent a paradigm shift in environmental monitoring and packaging technologies, allowing to assess air quality, formation of byproducts in closed environment, and the barrier properties of the polymers. To this end, all-polymer distributed Bragg reflectors represent a promising approach towards a reliable and cost-effective transduction of chemical stimuli, and effective colorimetric label-free selective detectors. We show selectivity attained by specific interactions between the polymer and the analytes. Such interactions drive the analyte intercalation through the polymer structure and its kinetics, converting it in a dynamic optical response which is at the basis of the Flory-Huggins photonic sensors. The multivariate analyses of the response kinetics also allow to


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distinguish binary mixtures. Additionally, we demonstrate that such optical response can be used to esteem the diffusion coefficients of small molecules within polymer media via simple UV-Vis spectroscopy retrieving data comparable to those obtained with state-ofthe-art gravimetric procedures. Last, we assess the figures of merit of the sensors in terms of lower detection limit, sensitivity and reversibility, demonstrating that such devices can pave the way to an innovative, simple, and low-cost detection method integrable to in-situ assessment of barrier polymers used for the encapsulation of optoelectronic devices, food packaging, and goods storage in general.

Introduction

Currently, barrier properties of polymer thin films to vapors and gas are assessed via gravimetric1 and pressure decay methods,2-3 or by optical techniques based on microscopy4 and infrared absorption,5 which remained substantially unchanged for the last few decades. These methods need dedicated equipment and cannot be performed

in-situ. In this scenario, research for new low-cost, simpler, and portable technologies to gather lab-on-chip devices is strongly pursued. In these regards, sensors based on the


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optical response of polymer distributed Bragg reflectors (DBR) represent a possible revolution in the field due to the high responsivity to analytes in the vapor phase.6-8 DBRs are planar photonic crystals made of media with different refractive index stacked to form a dielectric lattice. The periodic modulation of the refractive index in the DBRs leads to the generation of frequency regions for which light propagation is forbidden. These frequencies are called photonic band gaps (PBGs), and are easily detectable via simple reflectance or transmittance spectroscopy.9-15 In analogy with the energy-gap of semiconductors, the PBG properties depend on the lattice structure. Then, dielectric contrast among the lattice components, lattice parameters, and the number of layers affect the optical features generated by the DBR photonic structure.16 Intuitively, a perturbation of these parameters, affects the entire photonic crystal spectrum, and in turn its variation can be related to the stimuli such as pressure variations,17 chemical analytes,7-8, 18-23 and pH.24-26

Unlike inorganic meso-porous DBR sensors9-10,

21, 27-28

and other porous photonic

crystals,29-30 polymer structures often play as dense membrane and molecular diffusion


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is ruled by the analyte solubility in the polymer matrices, making them interesting disposable sensors.7 A comparison between these structure can be found in Ref.8,

10

.Therefore, to diffuse within the structure and affect the DBR optical response, the analyte needs to be solubilized within the dense polymer and then diffuse through it. The molecular species must then have chemico-physical affinity with the dense matrix. Such affinity can be defined by the Flory-Huggins parameter for the analyte-polymer pairs (𝜒𝐻 𝐴𝑃, neglecting entropic contribution), which can be expressed as a function of analyte molar volume (𝑉𝑀) and of the solubility parameter for the pairs. In turn, the latter is defined from the Hildebrand parameters of the two components of the mixtures (𝛿𝑃, 𝛿𝐴) as Δ𝛿2 = (𝛿𝑃 ― 𝛿𝐴)2, and expresses the difference of cohesive energy between the analytes and the polymers, the smaller is Δ𝛿2, the larger is the solubility.31-32

Δ𝛿2

𝑀 𝜒𝐻 𝐴𝑃 = 𝑉 𝑅𝑇

(1)


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where R is the gas constant and T the temperature. For this reason, all-polymer DBR can be called Flory-Huggins photonic sensors (FHPS).10 Then, the choice of suitable polymers as active media makes them efficient detectors for the label-free identification of a variety of analytes in the vapor phase, and allows to extend the method to a large amount of chemical species, including water, toxic and carcinogenic volatile organic compounds and even perfluorinated species,6, 8, 33-37 paving the way for a new generation of disposable photonic sensors with novel capabilities and broad band selectivity.

In this work, we propose an original proof of concept FHPS made of commodity polymers for the evaluation of the diffusion coefficients of molecular species into polymer matrices and for their discrimination to assess air quality. We demonstrate for the first time that DBRs-based disposable FHPS made of polystyrene (PS) and cellulose acetate (CA) discriminate between air enriched with different short chain alcohols, including methanol (MeOH), Ethanol (EtOH), 1-propanol (1POH), 2-propanol (2POH), and 1-butanol (BuOH), and their binary mixtures and show sensitivity and lower detection limit below the part per million. These analytes were chosen for two reasons. First, their discrimination is


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extremely challenging since the molecules have similar molecular weight and present comparable Van der Waals volumes, polarity, hydrogen bonding, and volatility. Second, the ability to disentangle these alcohols and to study polymer barrier properties is of extraordinary importance to prevent toxic effects. Indeed, while EtOH is widely used in food industry, MeOH, which is a byproduct of EtOH fermentation, generates toxic effects in both the acute and the cronic forms38 due to its metabolization to formaldehyde and formic acid.39-40 Moreover, 1POH and 2POH share a similar structure, physical properties, and applications, but 2POH is less toxic than 1POH and finds applications in many antibacterial and personal care products.40 Similarly, BuOH, which is interesting for biofulel,41 shows a very low toxicity, but causes eye and skin irritation and is harmful if inhaled.40

Working principle of FHPSs

The FHPSs under investigation are made of 31 alternated layers of PS and CA, as sketched in Figure 1a. Such structures provide a typical optical response, shown as a black line in the reflectance spectrum of Figure 1b. The spectrum displays two maxima of


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reflectance at 845 nm and 43 nm. These features are assigned to the FHPS PBG and to its second order replica. When the FHPS is placed in air polluted with a volatile molecular compound, the molecules permeate into its structure, and progressively swell the PS, the 6-8 The interaction between the polymer and the CA or both the layers, depending on 𝜒𝐻 𝑎𝑝.

analyte results in a variation of the thickness and refractive index of the layers, and thus, of the light optical path (Figure 1a). In turn, such variation affects the PBG spectral position, according to

𝜆𝑃𝐵𝐺 = 2 𝑑𝐿 + 𝑑𝐻 𝑑𝐿𝑛2𝐿 + 𝑑𝐻𝑛2𝐻

(2)

where 𝑑 and 𝑛 are the thickness and the refractive index of the low (𝑑𝐿, 𝑛𝐿) and high refractive index layers (𝑑𝐻𝑛𝐻). Indeed, the red line spectrum of Figure 1b shows that after 90 min of exposure to MeOH (see the Experimental section) the first order PBG of the FHPS red-shifts of about 100 nm to 940 nm, while the second order PBG shifts of about 50 nm to 475 nm.42


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Figure 1c illustrates the dynamics of the optical response as a contour plot where the exposure time and the wavelength are represented as the y- and as the x- axis respectively, while the reflectance intensity is reported as a color code. As mentioned above, the first and second order PBGs are initially located at 845 nm and 430 nm, respectively. These features are visible in red-tones, while the spectra background is represented in blue tones. The red-shift of the PBGs is initially very fast, and reaches ~80% of the final value within the first 10 min of exposure. Then, the dynamics slows down, and the system reaches the equilibrium in 90 min. To highlight the temporal evolution of PBG spectral positions, in Figure 1d we show the maximum of intensities of each spectrum, which correspond to the two spectral features, versus the exposure time. These data represent then the dynamic evolution of the first (blue line) and second (green line) order PBGs after normalization by the respective value of shift observed at the steady-state (~90 min, 1∞ = 100 nm and 2∞ =50 nm for the first and second order PBG respectively).


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The dynamic responses of Figure 1d are similar to the gravimetric sorption curves retrieved from diffusive processes in polymer slabs.43 Indeed, the penetration of a molecular species into the polymer films and their possible swelling affect both the refractive indexes (𝑛𝑃𝑆 and 𝑛𝐶𝐴) and the geometrical thicknesses (𝑑𝑃𝑆 and 𝑑𝐶𝐴) of the layers composing the FHPS. In turn, their optical thickness (𝑛 ∗ 𝑑) is modified. Because for vapor analytes ∆𝑑/𝑑(0) ≫ ∆𝑛/𝑛(0),7,

44-47

we can assume that the refractive index

variation is negligible during the swelling process, 𝑛(0) ≈ 𝑛(𝑡) ≈ 𝑛(∞). Then, we can derive that the absorbed vapor mass, M(t), is proportional to the variation of thickness, which in turn affects the PBG spectral position.

𝑀(𝑡) 𝑀(∞)

𝑑(𝑡) ― 𝑑(0)

÷ 𝑑(∞) ― 𝑑(0) ÷

∆𝜆1(𝑡) ∆𝜆2(𝑡)

∆𝜆1(∞),∆𝜆1(∞)

(3)

where 𝑀(∞) is the mass of the molecular species permeated through the polymer film at the equilibrium obtained for an infinite exposure time (𝑡 → ∞), 𝑑 = 𝑑𝑃𝑆 + 𝑑𝐶𝐴 , ∆𝜆1and ∆𝜆2


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are respectively the spectral shift of the first and second order PBGs at time t and at the equilibrium.

Figure 1: (a) Scheme of the intercalation of MeOH into a PS: CA FHPS where only CA swells, (b) reflectance spectra of a DBR before (black line) and after (red line) 90 min of exposure in MeOH environment, (c) contour-plot of the temporal evolution of the FHPS


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spectra during the MeOH exposure, and (d) normalized profile of the first (blue line) and second (open dots) order PBGs spectral position during the exposure.

The sorption rate of a molecular species through a medium typically exhibits two regimes: a non-steady-state one, which is regulated by the diffusivity of the molecules within the polymer matrix, and a steady-state one, which depends upon their equilibrium solubility at saturation. Standard gravimetric methods consist in the exposure of a supported polymer film at constant temperature, volume, and at a defined initial vapor pressure. In such measurements, the dynamics of the pressure variation is correlated to the polymer mass intake, which determines the equilibrium solubility at the steady state and by the diffusivity at the non-steady state. For a thin polymer slab with thickness d, such that the diffusion of the molecular species cannot occour through the sides (one-dimensional diffusion), the variation of concentration c(z,t) along the z axis can be described by the second Fick’s law,

∂𝑐 ∂𝑡

( ) ∂2𝑐

= 𝒟∂𝑧2 , where 𝒟 is the diffusion coefficient, 𝑐 is the concentration

of the molecular species within the slab and 𝑧 is the diffusion distance. The boundary


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conditions are 𝑐(z,t) = 0 at 𝑡=0 for each z, then at 𝑡 > 0 the surface concentration is equal to the equilibrium one, and

∂𝑐 ∂𝑧

= 0 at 𝑧 = 𝑑 for each t. Following this approach, one can

espress the mass intake as decribed by Crank:43, 48

𝑀(𝑡) 𝑀(∞)

∆𝜆(𝑡)

= ∆𝜆(∞) =

𝐷𝑡 𝑑2

(

∞

𝜋 + 2∑𝑛 = 1( ―1)𝑛𝑖𝑒𝑟𝑓( 2

)

𝑛𝑑 𝐷𝑡)

(4)

where 𝑖𝑒𝑟𝑓 is the complex error function.43 For short exposure time, where 𝑀(𝑡) 𝑀(∞) < 0.2 the second term of Equation 4 can be neglected, and:6,49

∆𝜆(𝑡) ∆𝜆(∞)

2 𝒟 𝜋

=𝑑

(5)

𝑡

Therefore, 𝒟 can be evaluated from the angular coefficient of the linear part of the sorption curve of Figure 1d, reported as ∆𝜆(𝑡)/∆𝜆(∞) vs.

t . This approach assumes constant

thickness and refractive index of the polymer slab. If we consider the early stages of the uptake process for which the thickness variation is below 20% of the total value, from Equation 5 we can derive Deff, with an error below 10%.8 When the polymer-analytes solubility can be neglected one can consider the analyte diffusion as an imbibition process of capillaries and porosity within a non-dense matrix.50-52


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Results and Discussion

Optical Assessment of Deff In a previous work, we demonstrated that the kinetics of the intercalation process described so far depends strongly on the interactions between the polymers and the analyte.8 Indeed, parameters like the analyte Van der Waals volume, polarity, and the fomation of hydrogen bonds affect the analyte permeation rate in the polymer layers, their swelling degree, and in turn the optical response of the FHPS. Then, the possibility to relate the permeation kinetics of the molecular species to the simple FHPS optical response paves the way for developing a new powerful tool capable of determining ina simpe way vapor diffusivity into polymer multilayers, also suitable for in-situ measurements without chemical functionalizations. Indeed, these systems are also efficient label-free selective sensors for vapor analytes, that could make colorimetric sensors suitable for safety devices in the food industry, industrial air pollution, households, and offices.8


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As a proof-of-concept, beside MeOH, we investigated a chemical series of short chain alcohols with chemico-physical properties scaling with the number of carbons. To evaluate the optical response of the sensors, a FHPS made of 31 layers of PS and CA was divided into five portions, which were exposed to the alcohols mentioned above in a closed environment at room condition to simulate real operating conditions. The evolution of the FHPS spectrum was sampled at a given frequency (see Methods section). The collected spectra are reported, described and analysed in Supporting Information Figure S1. These spectra and the retrieved optical dynamic responses show that the rate of the permeation process is characteristic and alcohol-dependent, thus allowing their label-free recognition.

Figure 2 shows the optical sorption curves retrieved from the contour plots for the response to the five alcohols reported in Supporting Information Figure S1. The curves show two main behaviors; the first one, which characterizes the sorption of MeOH and EtOH, consists in a linear increase of the analyte intake followed by the steady-state regime. The second, observed for 1POH, 2POH and BuOH, is characterized by the


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presence of discontinuities in the PBG position that occurs both at the steady and at the non-steady state regimes. In agreement with previous findings, while in the case of MeOH and EtOH the swelling of all the FHPS layers occur almost simultaneously, in the case of alcohols with larger steric hindrance, the intercalation is slower, and the polymers are gradually swollen from the top layer of the FHPS in contact with air to the bottom one in contact with the substrates.7-8 The presence of swollen and un-swollen layers breaks the DBR order destroying the condition that generates the PBG and its optical response and creating those discontinuities. Conversely, discontinuities at longer exposure time, can be assigned to relaxation and rearrangement of the polymer chains from stresses associated to the large swelling induced by the molecule intercalation, and has already been reported in literature.53


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Figure 2: Normalized spectral profile FHPS first order PBG spectral position versus the square root of the exposure time to: (a) MeOH, (b) EtOH, (c) 1POH, (d) 2POH, and (e) BuOH.


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The optical sorption curves can be used to assess the effective diffusion coefficient of the analytes in the whole polymer composite (𝒟𝑒𝑓𝑓). According to Equation 5, one needs to evaluate the angular coefficient of the sorption curves of Figure 2, and the initial thickness of the FHPS at 𝑡 = 0. The latter was estimated modelling the FHPS reflectance spectrum8, 34, 54

as described in Supporting Information Figure S3, the PS and CA thicknesses are

respectively 160 nm and 111.5, for a total FHPS thickness of 4.3 x 10-4 cm. The slope of the optical sorption curves has then been retrieved within their interval of linearity for values of

∆𝜆(𝑡) ∆𝜆(∞)

< 0.2 (Figure 2), while the value of ∆𝜆∞ was evaluated as the average ∆𝜆

in the plateau region of the curves reported in Supporting Information Figure 1 a”-c”.

Table 1: Steric and chemico-physical parameters of the species and retrieved 𝒟𝑒𝑓𝑓.

LOD (mg/l)

𝝌𝑯 𝒆𝒇𝒇

Sensitivity

δ (MPa1/2)

∆𝝀∞

𝓓𝒆𝒇𝒇 (cm2 min-

(mg/l at LOD -

32, 55-57

(nm)

1)

mg/l

𝒗 (Å𝟑)31 in

saturated air)

MeOH

67.6

29.6

1.22

100

2.5*10-8

29

19.5 – 0.8


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v

EtOH

97.1

26.1

0.81

141

7.9*10-10

12

15.6 – 0.9

1POH

124.9

24.4

0.64

180

3.8*10-11

12

6.1 – 2.9

2POH

127.5

23.6

0.56

182

2.2*10-11

6

13.4 – 3.5

BuOH

151.9

23.3

0.65

220

4.2*10-11

64

6.3 – 1.0

PS

-

18.7

-

-

-

CA

-

27.2

-

-

-

= Van der Waals volume, δ = Hildebrand parameter,

parameter, ∆λ = PBG shift at the equilibrium, 𝒟 ∞

eff

=

= effective Flory-Huggins

χHeff

effective diffusivity within the DBR.

The calculated values of 𝒟𝑒𝑓𝑓 are summarized in Table 1 and Figure 3a, where the data are also compared with results available in literature for MeOH and EtOH.58-59 The diffusivity of MeOH in CA has been reported as 8-9 x 10-8 cm2s-1 while for PS it is 1.4 x 10-8 cm2s-1 (at 55°C),25,37 in full agreement with the effective values retrieved for our composite PS-CA DBR. For EtOH, the literature proposes 1-7 x 10-10 cm2s-1 for CA and 9 x 10-10 cm2s-1 (at 55°C) for PS,58-59 again in agreement with our data (see Figure 3).


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Concerning the other alcohols, 1POH shows diffusion coefficient of 3.8 x 10-11 cm2s-1, for 2POH we retrieved 2.2*10-11 cm2s-1, while for BuOH 4.2*10-11 cm2s-1.

Figure 3: (a) Comparison between the retrieved value of 𝒟𝑒𝑓𝑓 (Equation 5) for the five alcohols with literature data for CA and PS,58-59 and effective Flory-Huggins parameter (


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𝜒𝐻 𝑒𝑓𝑓). (b) Comparison of the FHPS ∆𝜆∞ for five alcohols and the alcohol Van der Waals volumes. (c) Solubility parameters for the five alcohols with PS and CA.

The behavior of the sorption curves, and in turn 𝒟𝑒𝑓𝑓, can be affected by the steric hindrance of the analyte molecules and by its chemico-physical affinity with the polymers. Figure 3a compares the values of 𝒟𝑒𝑓𝑓 with the effective FHPS-analytes Flory-Huggins parameters (𝜒𝐻 𝑒𝑓𝑓), calculated from Equation 1 considering the volume fraction of PS and CA within the DBR and neglecting the entropic contribution.31 The panel b of the same Figure compares instead the FHPS PBG shift at the equilibrium, with Van Der Waals volume of the analytes. Interestingly, the Van der Waals volume values are strongly correlated to ∆𝜆∞, and then to the polymer swelling (Equation 2). Conversely, there is a strong correlation between 𝒟𝑒𝑓𝑓 and 𝜒𝐻 𝑒𝑓𝑓, and in turn with the solubility parameters between the polymers and the analytes ( Figure 3 a and c).32 Then, the only requirement to achieve selectivity is that the investigated analytes provide different 𝒟𝑒𝑓𝑓, that means different solubility, and then Flory-Huggins parameter in one of both the polymers.


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Figure 3c shows that, within the alcohols, the solubility of CA, is larger than that of PS and it is roughly constant. Conversely, for PS ∆𝛿2 decreases with the molecular weight of the alcohols, affecting also χHeff. These data prove that the decrease of 𝒟𝑒𝑓𝑓with the increase of the analyte molecular weight (panel a) is not linked by their solubility in the CA layers but in PS, which act as reservoir slowing down the analyte diffusion. Then, in the case of the most polar alcohols, CA plays as the sole active medium undergoing swelling, while the interaction between the analyte and PS can be considered negligible. This interpretation is also confirmed by the measurements of thickness variation upon MeOH exposure of single PS and CA films cast on glass substrates reported in Supporting Information Figure S2. When the films are exposed to MeOH vapors, CA swells dramatically, while PS thickness is not affected.

Label-Free Selectivity This simple and powerful optical method for the determination of 𝒟𝑒𝑓𝑓 also applies to the discrimination of the analytes. Figure 2 and 3 show that even molecules with very similar


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structure and properties, such as 1POH and 2POH, provide a very different optical sorption kinetics and then different values of 𝒟𝑒𝑓𝑓. We would like to stress that the optical sorption curves, and more generally the dynamic of the overall FHPS optical responses are characteristic fingerprints of the analytes, that lead label-free discrimination and detection using simple DBR structures and low-cost optical setup.7-8 Moreover, the large spectral shifts make possible to discriminate the analytes also by the analysis of the spectral response at set times. This simple signal transduction allows colorimetric safety devices for air pollution suitable to detect the presence of an analyte in the vapor phase by un-trained users. As an example, Figure 4 shows the experimental spectra of the FHPS collected before and after 50 min of exposure to the five alcohols. While the sample initially shows two PBGs at ~845 nm and 430 nm, when exposed to MeOH, the PBGs width increases, and their spectral position shift to 931 nm and 474 nm, respectively. For EtOH the shift is smaller and the PBGs reaches 919 nm and 465 nm. 1POH instead provides a reduction of the peak width which, after 10 min of exposures moves to 861 nm and 436 nm. Concerning BuOH, the alcohols with higher molecular weight, its intercalation induces disorder and inhomogeneous broadening of the PBGs, which


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reaches maxima of intensities at 860 nm and 425 nm after the exposure. The dynamics of these spectral responses are shown in Figure 4 b-d for the five alcohols as contour plots. These patterns, which are described in detail in Supporting Information Figure S1, represents the characteristic polymer-solvent kinetic interaction, and can be unambiguously considered the fingerprint of the analyte detected with the specific FHPS.

The simple colorimetric analysis and the assessment of the entire diffusion kinetics represent an effective method for the discrimination of the analytes. On the other hand, analyzing the entire curves is rather complex and does not allow to easily distinguish mixtures. To achieve this goal and simplify data interpretation, one must consider that the dynamical optical responses are a typical playground for multivariate analyses, and then a principal component analysis (PCA) can be applied to disentangle pure analytes and their mixtures. Figure 4g shows that the response can be described using only two variables (principal components PC1 and PC2) instead of the entire dynamics. We can indeed easily distinguish the five alcohols (squares) and binary mixtures of MeOH and


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BuOH. Such result represents a promising approach for the identification of complex mixtures in real systems.

Figure 4: (a) Reflectance spectra of the FHPS sensor before (black line) and after 50 min of exposure to MeOH (red line), EtOH (green line), 1POH (blue line), 2POH (cyan line),


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and BuOH (magenta line). Contour-plots of the dynamic FHPS response for: (b) MeOH, (c) EtOH, (d) 1POH, (e) 2POH, and (f) BuOH. (g) Power component analysis of the DBR response for the five analites and for three binary mixtures containing MeOH and BuOH in different concentrations.”

Response time, sensitivity and lower detection limit To evaluate the quantitative figures of merit of the sensor, we exposed it to different concentrations of pure analytes in the vapor phase. The data displayed in Figure 5a were retrieved for a FHPS made of 5.5 bilayers with PBG tuned at 450 nm (See Supporting Information Figure S5), where the thinner layers and the smaller number of polymer films allows responses almost 20 times faster than those seen so far. The plot reports the response as the FHPS PBG shift obtained after only 5 min of exposure. The data were collected in concentration ranging from the analyte vapor pressure to the limit of detection (LOD) of the FHPS, that is the concentration inducing spectral shift as large as the spectrometer resolution (1.5 nm) within 5 min. Intuitively, increasing the set response time, it is possible to further decrease the LOD. Within the 5 min the LOD ranges from a


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minimum of 6 mg/l for BuOH to a maximum of 65 mg/l for 2 POH (see also Table 1). Table 1 helps to evince that for linear alcohols the longer the carbon chains, the smaller is the LOD. For non-linear 2POH instead, the very slow response induced by the steric hindrance of the molecules produces larger LOD as expected.

To evaluate the sensitivity of the devices we fitted the response of Figure 5 with a first order kinetics and calculated the minimum concentration variation detectable by our optical system, which corresponds to a PBG optical shift of 1.5 nm. Such data are reported in Figure 5b and shows that due to the exponential dependence of the PBG shift with increasing concentration, the sensitivity of the system varies for different concentration intervals in the range 0.8-16 mg/l from the largest concentration to the smallest concentration of MeOH. The values for the other alcohols are within this range. The sensitivity is in full agreement with data previously reported in literature for other polymer photonic crystal sensors.8, 10


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Figure 5: (a) PBG spectral shift collected after 5 minutes of exposure to different concentration of the five alcohols. The concentration ranges from the analyte vapor tension to the LOD of the sensor. (b) FHPS sensitivity at the different concentrations.

Last, concerning reversibility, Supporting Information Figure S5 shows that the optical response of the sensors is not fully reversible after several cycle of exposure and desorption. Such characteristics makes them very interesting as disposable sensors, similarly to the colorimetric quantitative test tubes already available on the market.60 On


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the other hand, the optical transductors are few millimeters in size and fully processed from solution using commodity polymers. Furthermore they can also be grown over square meters by coextrusion,61-64 a technique widely used in the industrial production of packaging,60, 65 making realistic the production of FHP sensors on large scale and at low costs.

Exploiting the fundamental thermodynamic of polymer-analyte mixtures allows then to engineer FHPSs affinity with degradation by-products or harmful chemical species. We envision their integration in smart packaging systems to monitor in-situ internal and external environments and assess diffusion of small molecules. This will also be possible thanks to small and compact detection systems already available on the market.66-67

Conclusions This work provides a versatile tool for the optical determination of the effective diffusion coefficients of vapor analytes within simple multilayered polymer distributed Bragg reflectors in excellent agreement with those reported in literature. The analysis of the kinetics can also be related to the chemico-physical interaction between the analyte and


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the polymers allowing simple and label-free molecular recognition by FHPS engineered

ad hoc. The optical behavior of a DBR made by commodity polymers, which are easy to integrate in smart packaging devices, has been fully investigated during the exposure to five short chain alcohols. The study of the dynamic optical responses of the FHPS allows to retrieve sorption curves characteristic of the analytes that provides their diffusion coefficients and permits their discrimination. Since the FHPSs are sensitive to a large variety of molecular species, these proof of concept devices are promising for the development of sensors to be used on-site for the assessment of environmental pollution and for smart packaging.

Experimental

Sample Preparation: Polymer FHPSs were fabricated by spin-coating deposition of 31 alternated layers of cellulose acetate (Sigma-Aldrich, Mn = 50 000) and polystyrene (Sigma-Aldrich, Mw = 192 000) dissolved in 4-hydroxy-4-methylpentan-2-one and toluene,


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respectively. The solution concentration was ~3% w/w and the rotation speed during the deposition was kept at ~8000 RPM.

Characterization: For all the FHPS, reflectance data were collected with a homemade setup based on optical fibers. Reflectance data were collected with a reflectance fiber probe, an Avantes AvaSpec-2048 spectrometer (200−1150 nm, resolution 1.4 nm), and a deuterium − halogen Micropak DH2000BAL light source. More details are reported in Ref. 7-8

The optical response of the sensors during the exposure to methanol, ethanol, 1propanol, 2-propanol, and 1-butanol was measured at ~ 26 °C (Table 1), 1 atm and humidity within 65-80% in a closed container where a 0.5 mL of the analyte were previously placed to saturate the environment. The effect of different temperature, humidity and pressure on the sensor response were not tested. The concentrations of the analytes in the vapor phase are reported in Table 1. The optical response was recorded at set time intervals with the optical setup previously mentioned using a reflectance immersion probe.


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Spectra Modeling: The modelling of the FHPS spectrum (Supporting Information Figure S3) was performed with a MATLAB® homemade code based on the transfer matrix method as reported in previous works.7-8 We used the refractive index dispersion from literature54 as inputs and the layer thickness as fitting parameter.

Morphological Characterization: The morphology of the FHPS before and after MeOH exposure (See Supporting Information Figure S5) was done with a Nanosurf Atomic Force Microscope (AFM) on an area of 10μm x 10μm with a sampling rate of 512 points/line. We extracted root mean square roughness (σ) of the surface from the processed images and we measured a value of σ =1.9 nm for the unexposed DBR and σ=13 nm on the FHPS exposed to 5 cycles of MeOH vapor.

ASSOCIATED CONTENT Supporting Information Full FHPS response to vapor exposure, assessment of interacting and barrier media, Evaluation of FHPS layer thicknesses, Operative sensing conditions, Response of the FHPS to several cycles of exposures and desorption.


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Notes The authors declare no competing financial interest AUTHOR INFORMATION Corresponding Authors * E-mail: [email protected] # E-mail:

[email protected]

Author Contributions AS, DC and PL conceived the project and designed the experiments. CB worked on the sample fabrications. Optical characterizations were performed by CB, CM, and PL. Data were analyzed and modelled by AS, DC, GM, and PL. AS, DC and FBM coordinated the work. The manuscript was written through contributions of all authors. ACKNOWLEDGMENTS This work was found by the European Union’s Horizon 2020 research and innovation program Marie Sklodowska-Curie grant agreement No 643238 and by the Italian Ministry of University, Research and Instruction through the “Progetti di Ricerca di Rilevante Interesse Nazionale 2010−2011” Program (Materiali Polimerici Nanostrutturati con


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strutture molecolari e cristalline mirate, per tecnologie avanzate e per l’ambiente, 2010XLLNM3). ABBREVIATIONS BuOH, 1-butanol; 1POH, 1-propanol; 2POH, 2-propanol; CA, cellulose acetate; DBR, distributed Bragg reflector; EtOH, ethanol; FHPS, Flory-Huggins photonic sensor; MeOH, Methanol; PBG, photonic Band-gap; PS, polystyrene. REFERENCES (1)

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