1 Introduction

Smart labels and packaging monitor and convey information about items by incorporating technologies, such as indicators, sensors, visual displays, and data carriers. Key markets for smart labels include healthcare [1], food packaging [2], logistics, and authenticity labeling [3]. Figure 1 highlights the key advantages, disadvantages, and examples of commercial devices with different smart label form factors for sensing applications.

Fig. 1
figure 1

Summary of key advantages and disadvantages of different form factors of smart labels and packaging. Examples of commercial products for indicators, sensors with visual displays/indicators, and sensors with non-visual outputs

Full size image

A simple smart label consists of an indicator that has a direct change in color, intensity, or diffusion of color in response to a change in environmental conditions. Technologies have been developed to produce indicators that show response to temperature, impact, pH, humidity, light, and the presence of gases, chemicals, or biological species [1,2,3,4,5,6,7]. Indicators have the advantage of being simple and low cost. Smart labels based on indicators are commercially available using both reversible and irreversible systems [8,9,10,11,12,13,14]. But, if more precise detection is required it can be advantageous to use an electronic sensor. This form of monitoring system has a higher cost but can offer greater functionality and customization to the user. In this case, the sensor is integrated with a power source and other components, such as radio-frequency identification (RFID) [15,16,17], near-field communication (NFC) [15, 17], micro-processing units, or dynamic visual interface. Dynamic visual interfaces can be incorporated in the form of light-emitting diodes (LEDs) [18,19,20,21], electrophoretic displays (EPDs) [22], electrochromic displays (ECDs) [23], or liquid crystal displays (LCDs) [24]. However, LEDs and ECDs are reliant on ongoing power supply to maintain a visual color state. And while EPDs and some LCDs are bistable, their coloration is still electrically reversible, limiting their use in authenticity or tamper-proof applications where a truly permanent color change is the key selling point. Thus, it is of interest to develop an irreversible electrochemical indicator to use as a visual interface for smart labels and packaging.

One approach to developing irreversible electrochemical indicators is via in situ electropolymerization. These indicators are produced by solubilizing monomers in the electrolyte of a vertical device structure. The indicator is activated by applying a sufficient potential to electropolymerize the species in situ. If the monomers absorb in the ultraviolet region and the polymer absorbs in the visible region, the result is a transparent to colored optical transformation. A schematic of the indicator construction is shown in Fig. 2. With increasing electrochromic film thickness the contrast between the oxidized and reduced states decreases [25, 26]. The indicator is considered irreversible when the color states cannot be differentiated by the human eye. It is important to note that the term ‘irreversible’ here refers to the perceived optical properties of the indicator, not to the electrochemical properties of the indicator.

Fig. 2
figure 2

Schematic showing the construction of an irreversible electrochemical indicator

Full size image

The use of in situ electropolymerization to create irreversible visual indicators was first reported in a 2009 patent from Chromera Inc. [27]. The group of G. A. Sotzing has also produced a body of work exploring in situ electropolymerization as a simple and low waste method to manufacture reversible electrochromic displays [28]. The devices in this case were reversible since polymerization time was limited to 30 s, and the resultant films are only around 0.2 au in their oxidized state [29]. They investigated how a series of parameters including monomer species, monomer concentration, salt species, and salt concentration influence the overall device performance [26, 30]. For the in situ electropolymerization of poly(2,2-dimethyl-3,4-propylenedioxythiophene) (PProDOT-Me2), the photopic contrast could be optimized by modifying the composition of binary plasticizer mixtures and both the photopic contrast and polymerization charge density could be modified by tuning the cross-linking density of a poly(ethylene glycol) dimethacrylate gel network [31, 32]. Interestingly in another report of in situ electropolymerization, Zhao et al. produced a copolymer with EDOT and an anthracene derivative in a device using a polyethylene glycol (PEG) gel electrolyte and found that they could form a polymer in situ that they could not form in a solution of just the plasticizer, proposing that the in situ format may assist the film deposition process [33].

The properties of electropolymerized films are dependent on a range of environmental factors, including the nature and concentration of the monomer, solvent, and salt, as well as the substrate and the electrical perturbation (i.e., potentiostatic, potentiodynamic, galvanostatic) [34,35,36,37,38,39,40,41,42,43,44]. Generally, the deposition proceeds via oxidation of monomers at the electrode surface and subsequent oligomerizations, building up a high-density oligomeric region. Once the high-density oligomeric region is established in front of the electrode surface, film formation proceeds via nucleation and growth processes [45].

The goal of this work is to investigate how a polymer gel matrix influences the polymer electrodeposition process and to characterize this effect for in situ-polymerized irreversible electrochemical indicators. Gel polymer electrolytes (GPEs) combine the high ionic conductivity of liquid electrolytes with the robust mechanical properties of solid electrolytes, making them desirable for flexible devices [46,47,48]. This is initially studied in a 3-electrode electrochemical cell set-up using a platinum disk working electrode and then tested in flexible indicators with indium tin oxide (ITO)-coated poly(ethylene terephthalate) (PET) substrates.

PEDOT has been widely studied due to its ease of synthesis and unique physical and electrochemical properties. The monomer EDOT is colorless in solution, broadly commercially available, and has a low oxidation potential arising from the electron donating nature of its alkoxy substituents [49]. This, along with its solubility in the binary ethylene carbonate and propylene carbonate mixture used in our electrolyte, makes it a promising candidate for irreversible electrochemical indicators [50]. Further, the polymer PEDOT presents good stability in air and at higher temperatures, which is key to developing indicators that can be used in a broad range of environmental conditions [51, 52]. UV cross-linkable EO-PO-AGE terpolymer Zeospan 8030, structure shown in Fig. 3, was selected for the gel electrolyte as it has been used industrially for the production of electrochromic devices [53,54,55]. The terpolymer composition is 91% ethylene oxide, 6% allyl glycidyl ether, and has Tg =  − 57 °C, density − 1.16 g cm−3, and Tm = 40 °C [56, 57].

Fig. 3
figure 3

Chemical structure of EO-PO-AGE Zeospan 8030 polymer

Full size image

2 Methods

2.1 Materials and equipment

Ethylene carbonate (99%) and propylene carbonate (99.5%) were obtained from Sigma-Aldrich, lithium perchlorate (98%) was obtained from Alfa Aesar, photo-initiator Phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide was obtained from TCI Chemicals, and Zeospan 8030 was obtained from ZEON Chemicals. All chemicals were used as received.

For the electrolyte formulation, a PC:EC:LiClO4 ratio of 1:0.47:0.098 was maintained. The liquid electrolyte was mixed in a vial using a roll bar mixer. The gel electrolyte was formulated by incorporation of 9.95-wt.% Zeospan 8030 and 0.5-wt.% photo-initiator to the pre-mixed liquid electrolyte using a stainless steel blade mixer. Gel curing was performed with a LOCTITE 500-W mercury vapor bulb for 120 s. Electrochemical experiments were carried out on an AUTOLAB PGSTAT100N potentiostat. Optical spectroscopy was performed on an Agilent Cary 300 UV–Vis Spectrophotometer. Digital images were captured with an IDS Imaging UI-3590CP-C-HQ Camera and color corrected using an x-rite colorchecker included in frame for each photo (see Figure S1). The abbreviation ‘PC:EC:LiClO4 + 10% ZEO’ is used for the gel electrolyte, and the abbreviation ‘PC:EC:LiClO4’ is used for the liquid electrolyte.

2.2 3-electrode electrochemical cell

The working electrode was a Platinum disk electrode (1.8 mm dia.), which was polished to a mirror-like surface with 0.05-µm alumina slurry and rinsed in ethanol prior to each experiment. The counter electrode was a platinum mesh flag (1 × 1 cm2), and the reference electrode was non-aqueous Ag/Ag + in 0.01-M AgNO3 and 0.1-M TBAP/ACN filling solution (E1/2 for Fc/Fc+: 0.05 V). For experiments with liquid electrolyte, the working electrode was immersed directly in the monomer electrolyte solution. For experiments with gel electrolyte, a 0.1-cm2 cylinder of the gel electrolyte with monomer was cured directly onto the tip of the working electrode and then immersed into a liquid electrolyte solution with the same monomer concentration (see Figure S2).

2.3 Irreversible electrochemical indicator construction

Flexible indicators were fabricated using a vertical device architecture composed of two 80 Ω□−1 PET-ITO electrodes (Eastman, FLEXVUE) and a 220-µm adhesive spacer material. The working area is a 1 cm2 area circle. The electrolytes were drop cast and the indicators were manually sealed. Copper tape was added to improve the electrical contact to the power source.

3 Results

3.1 3-electrode cell

We used both potentiodynamic and potentiostatic techniques to investigate the electrodeposition of PEDOT in the liquid and gel electrolyte. Cyclic voltammetry is a potentiodynamic method where the potential of the working electrode is ramped linearly over time. The current response in cyclic voltammetry can be used to determine the oxidation potential of the monomer, the formation of a polymer film, and the electrochemical behavior of the deposited film.

The first scan of the cyclic voltammetric response of electropolymerization of 25-mM EDOT in liquid and gel electrolyte is shown in Fig. 4a. A rapid increase in the current density, ascribed to the initial oxidation and cross-coupling reactions of the monomer, occurs at a lower potential (0.7 V vs. Ag/Ag+) in the gel electrolyte than in the liquid electrolyte (0.8 V vs. Ag/Ag+). The same cyclic voltammetric protocol was conducted in monomer-free electrolytes to investigate the electrochemical response of the electrolyte, and the first scan is shown in Fig. 4b. For the gel electrolyte, a 0.14-mA cm−2 peak is observed at 0.62 V vs. Ag/Ag+. The same peak is observed in the gel electrolyte with monomer and is attributed to the polarization of the gel matrix.

Fig. 4
figure 4

First scan of cyclic voltammetry in liquid and gel electrolytes with 25-mM EDOT monomer (a) and without EDOT monomer (b). Scans 1–10 of cyclic voltammetry in liquid (c) and gel (d) electrolyte with 25-mM EDOT. Scan rate is 0.05 V s−1

Full size image

Scans 1–10 of the same cyclic voltammetric experiment shown for electropolymerization in liquid and gel electrolytes are shown in Fig. 4c, d. The appearance of broad oxidation and reduction peaks below the oxidation potential of the monomer indicates that a polymer film is deposited from both samples. In the liquid electrolyte, the increase in current density for the polymer’s cathodic redox peak (~ 0.090 mA cm−2) is greater than in the gel electrolyte (~ 0.054 mA cm−2) for consecutive scans. The increase in cathodic redox charge density for each cycle is a measure of the increase in the number of rechargeable redox sites, suggesting that more polymer film is formed from the liquid electrolyte after 10 cycles than from the gel electrolyte.

Potentiostatic techniques can be used to investigate the nature of the deposition process during electropolymerization. The electrical protocol and an experimental current–time transient for the deposition of EDOT using the potential step method is shown in Fig. 5.

Fig. 5
figure 5

Electrical protocol for the potential step method, and current–time transient of 25-mM EDOT in liquid electrolyte at 800-mV applied potential

Full size image

The following regions are observed:

  • A–B: The pre-conditioning potential is applied (less than the oxidation potential of the monomer). A sharp peak is observed due to the charging of the double-layer capacitance.

  • B–C: The film-forming potential is applied. Oxidation of monomers begins at the electrode surface. Soluble oligomers are generated forming a high-density oligomeric region in front of the electrode surface. This region is characterized by a diffusion-controlled j ~ t−1/2 relationship. The time at the current minimum is referred to as the induction time (τ).

  • C–D: After the induction time, nucleation and growth on the electrode surface produce an increase in the current density. A maximum (t(jmax), jmax) and decrease in the current density is indicative of nuclei overlap, reducing the effective surface area and/or the transition to a diffusion-controlled process [41, 42].

  • D: Subsequent current density steps or peaks suggest a multi-step or layer-by-layer electrodeposition process [58].

Figure 6a, b show the current–time transients during the film-forming region for films formed over a range of potentials in the liquid and gel electrolyte. The general features of the deposition of EDOT in liquid electrolyte in this study are consistent with those previously reported in the literature [41, 58]. After 300 s in the gel electrolyte, a visible polymer film is deposited on the electrode at potentials above 640 mV, whereas in liquid electrolyte a visible polymer film is only deposited at potentials above 720 mV. This amounts to an 80-mV reduction in the potential required for film formation. In the liquid electrolyte, above 790 mV the transients show a second peak in the current density, suggesting a multi-step deposition process. However, in the gel electrolyte, even at high potentials, the process remains a single step and the current density gradually decreases to a low current density plateau.

Fig. 6
figure 6

Current–time transients for electropolymerization of 25-mM EDOT in liquid (a) and gel (b) electrolyte over a range of potentials. The interval between transients is 10 mV. Dashed line indicates that no visible polymer is formed on the electrode surface. (c) Charge consumption at 300 s for electropolymerization of 25-mM EDOT in liquid and gel electrolytes. (d) T(jmax) vs. applied potential for electropolymerization of 25-mM EDOT in liquid and gel electrolytes

Full size image

Figure 6c shows the total charge consumed by the samples over the 300-s interval. There is a clear cross-over point where at potentials less than 770 mV more charge is consumed in the samples with the gel electrolyte, whereas at potentials greater than 770 mV more charge is consumed in the samples with the liquid electrolyte. Although we cannot exclude that other side reactions may contribute to this charge, it suggests that more polymer is formed from the gel electrolyte at lower potentials. The cross-over point in the charge consumption shifts to higher potentials for shorter time periods (see Figure S3). Additionally, at the same potentials, a rapid increase in current density to the plateau (T(jmax), jmax) occurs faster in the gel electrolyte than in the liquid electrolyte, see Fig. 6d. This could indicate that initial polymer deposition and transfer to diffusion-controlled processes occur faster in the gel electrolyte compared with the liquid electrolyte. Overall, the results from the 3-electrode cell suggest that the gel matrix produces a faster polymerization rate at lower potentials and short time periods compared to the liquid electrolyte.

A hypothesis for the observations concerns the nature of monomer and oligomer diffusion. In a liquid phase, species are free to migrate in all directions, whereas in a gel phase, the migration of species is limited by a 3-dimensional polymer matrix. As the relative ratio between the size of the migrating species to the pore size of the gel increases, movement is limited by steric hindrance and may require uncoiling or reputation [59]. This phenomenon is commonly exploited in gel electrophoresis.

In the context of the current study, the mobility of monomers and oligomers through the gel matrix is expected to decrease with growing chain length. Compared to monomeric species which may diffuse to and from the electrode surface, oligomers formed near the electrode surface would encounter more steric hindrance limiting diffusion back to the bulk. This potential trapping effect of oligomers near the electrode surface may facilitate the formation of the high-density oligomeric region, which is critical for the early stages of the deposition.

3.2 Irreversible electrochemical indicators

PET-ITO-based irreversible electrochemical indicators were constructed to determine whether the trends identified in the three-electrode cell were also observed in a device format. Triplicates of liquid and gel electrolyte indicators were activated over the range of potentials from 2.3 to 3.2 V. The indicators were activated by applying a sufficient potential to electropolymerize the monomers in situ. The transmittance of the indicators was measured at 555 nm, the peak of human photopic vision, over the duration of the activation. Figure 7 shows the results of this experiment for selected time periods. After 300 s of activation at the specified potential, 0 V is applied for 5 s to short circuit the device. The decrease in the transmittance when the indicator is short circuited is attributed to the electrochemical reduction of the PEDOT film on the working electrode. The transmittance of the indicator when it is short circuited is important to consider since the indicators will drift to this color state when the activation potential is removed.

Fig. 7
figure 7

Optical transmittance at 555 nm of PET-ITO indicators with in situ electropolymerization of 25-mM EDOT in liquid (a) and gel (b) electrolyte over a range of potentials. SC is the transmittance of the indicators after they have been short circuited for 5 s following the 300-s activation

Full size image

The overall color change in the indicators is a combination of the color change on the working and counter electrode (see Figure S4). In this case, the color change on the counter electrode is attributed to the reduction of the ITO coating on the PET-ITO substrate. While this reaction is considered to be parasitic in traditional electrochemical devices [60,61,62], it improves the change in transmittance and opacity for these irreversible indicators. The anomalous increase in the transmittance for the liquid electrolyte indicators in the range of 2.6–3.0 V and for the gel electrolyte indicators in the range of 2.9–3.2 V is primarily derived from trends in the transmittance of the ITO reduction on the counter electrode (see Figure S5).

For all potentials measured, the gel electrolyte indicators measure a higher change in transmittance after 300 s of activation. The difference in transmittance is greater at lower potentials. At 2.3 V, the gel electrolyte indicators have a change in transmittance of 40.1% after 300 s, compared to 10.9% in the liquid electrolyte indicators, see Fig. 8a. The standard deviation between the triplicates was higher in the liquid electrolyte indicators than in the gel electrolyte indicators. Figure 8b shows the coloration efficiency of the irreversible electrochemical indicators as a function of potential. The coloration efficiency (η) is the change in optical density (ΔOD) per unit of charge (Q) at a specific dominant wavelength. This is defined in Eq. 1

$$\eta = \Delta {\text{OD}}/Q$$
(1)

where the coloration efficiency is calculated for the coloration of the electropolymerization activation so ΔOD = [Tinacivated/Tactivated]. The coloration efficiency is higher for the gel electrolyte indicators than the liquid electrolyte indicators at all of the potentials tested.

Fig. 8
figure 8

Change in transmittance (a) and coloration efficiency (b) of PET-ITO indicators with in situ electropolymerization of 25-mM EDOT in liquid and gel electrolyte over a range of potentials. Coloration efficiency and change in transmittance are measured at 555 nm for 300 s of activation. (Color figure online)

Full size image

Digital images of liquid and gel electrolyte indicators activated at 2.3, 2.5, and 3.0 V are shown in Fig. 9. In addition to faster activation, the indicators with gel electrolyte show greater visual homogeneity. At 2.3 and 2.5 V, the liquid electrolyte indicators have color change primarily in the center of the working area, as opposed to the gel electrolyte indicators which have visually uniform color change across the full working area.

Fig. 9
figure 9

Color-corrected digital images of PET-ITO indicators with in situ electropolymerization of 25-mM EDOT activated at 2.3, 2.5, and 3.0 V. Images are taken of the pristine indicator before activation, during activation at 10-, 60-, 120-, 180, and 300-s time points, and when the indicator is short circuited (SC) after 300 s of activation. (Color figure online)

Full size image

4 Conclusion

In this work, we studied the influence of a polymer gel electrolyte on the electropolymerization of EDOT and demonstrated the use of in situ polymerization for irreversible electrochemical indicators. We found that the addition of UV cross-linkable EO-PO-AGE terpolymer as a gel matrix lowers the overpotential, increases the change in transmittance, and improves the coloration efficiency for film formation for irreversible electrochemical indicators. These initial observations indicate the need for further studies investigating the effect of gels on the electropolymerization process and highlight the potential to optimize electrolytes specifically for this form of irreversible electrochemical indicator.