Liquid crystal-based materials, in which liquid crystal molecules are confined and ordered in compartments, are dynamic materials yielding a variety of optical textures that can be tuned as a response to physical and chemical stimuli. While nematic and smectic-based gel materials have been reported as dynamic optical sensors to report volatile organic compounds (VOCs), chiral systems are less explored despite having the potential to yield extremely rich optical landscapes. Here, we report for the first time the confinement of chiral liquid crystal formulations by an interface formed by ionic liquid molecules. The resultant self-assembled ionic liquid/liquid crystal droplets are simultaneously immobilized on a gelatin matrix. The droplets feature a rich variety of unique topological states. We explored, by means of polarizing optical microscopy, the various droplet optical textures and categorized them with regard to their relative chirality parameter. We further investigated their optical response in the presence of gas analytes and discussed their potential utilization as dynamic liquid crystal-based optical VOC sensors. The newly generated soft materials with semi-selective VOC sensing capabilities can be further utilized in arrays of liquid crystal-based gas sensors for the analysis of complex gas samples using artificial olfaction approaches.

Chiral nematic liquid crystals (or cholesteric liquid crystals, CLCs) organize in helicoidal superstructures of the director.1 Locally, there is a nematic packing of the molecules, that is the molecules are oriented along a preferred direction. However, the director (n̂)—a unit vector related to the preferred molecular orientation—twists continuously around a perpendicular helical axis. The director periodicity is defined by the pitch (p), the distance over which the director rotates by 2π through the bulk. Considering that all the liquid crystal orientational structures can be easily influenced by external physical and chemical factors such as electric/magnetic fields, surface effects, and chemical or biological species, they are regarded as effective platforms for conveying molecular events,2–11 as in chemical or biological sensing.12–21 

Confining liquid crystals in restricted geometries such as droplets, a prerequisite for technological applications, allows for controlling the director orientation in order to achieve thermodynamically stable structures.15,22–26 In fact, a rich variety of structures and associated functionalities can be achieved through the delicate balance between the bulk elastic energies and the surface anchoring. The free elastic energy density of a bulk nematic is defined according to the Frank–Oseen equation27 as

FFO=12K11(·n̂)2+12K22(n̂·×n̂)2+12K33(n̂××n̂)2,
(1)

where K11, K22, and K33 are the Frank elastic constants, which are associated with the resistance of the material to splay, twist, and bend deformations, respectively. In the case of a CLC, Eq. (1) is modified by introducing the helical wave vector q=2π/p, to consider the spontaneous twist of the director, as follows:

FFO*=12K11(·n̂)2+12K22(n̂·×n̂+q)2+12K33(n̂××n̂)2.
(2)

For curved geometries, a fourth term, which is associated with the surface interaction contribution, should be added to Eqs. (1) and (2), namely, the saddle-splay term,

12K24·[n̂·n̂+n̂××n̂].
(3)

The saddle-splay term is quite important28 for the description of curved geometries since it decreases the free energy, as for most studied materials K24 > 0, and can heavily contribute to a rich phenomenology such as stabilizing textures in nematic droplets,29 or the induction of periodic stripe patterns in thin nematic films with hybrid anchoring conditions.30 

On the other hand, the surface anchoring is influenced by the bounding interface, since liquid crystal molecules anchor in a preferred orientation relative to the interface. The anchoring energy, which refers to the energy needed to divert the director from its preferred direction, is quantified by a parameter W. Rapini and Papoular31 have proposed an expression to account for the anchoring contribution to the surface free energy,

FS=12W(n̂·ê)2dS,
(4)

where ê is a unit vector along the preferred orientation.

Spherical confinement of CLCs coupled with their inherent twist organization gives rise to a plethora of distinct defect topologies32–34 as curvature costs energy. A topological defect is an arrangement of the order parameter, that is the quantification of the degree of molecular orientation relative to the director, which cannot be transformed continuously into a defectless state. They can exist either as points or lines and can be located either at the surface or within the droplet. Overall, defects can provide insight into the orientational structure and properties of liquid crystal materials, and most importantly, they can be easily perceived by means of polarized optical microscopy (POM).

Two critical parameters heavily influence the director profiles in CLC droplets: the boundary conditions and the ratio of the droplet diameter d to the helix pitch p. The relative chirality parameter N = 2d/p refers to the number of π turns of the director along the droplet diameter. The most commonly observed configurations for degenerate planar boundary conditions are the twisted bipolar23,33,35 [Fig. 1(a)] and the radial spherical structures34,36,37 [Figs. 1(b)–1(d)]. Typically, twisted bipolar director profiles are more energetically favorable for low chirality and smaller droplets. It is characterized by two-surface defects (named boojums) located in diametrical positions, similar to the bipolar structure of achiral nematic droplets, and a helical twist of the director, in a direction perpendicular to the line that connects the boojums.23,38 The Frank–Pryce structure, also known as the radial spherical structure or the spherulitic texture, exhibits a pair of disclination lines manifesting from the droplet center, which spiral intertwined toward the droplet surface and end in two-surface defects close to each other.23,32 Upon POM examination along the disclination pair, a spiraling texture is observed [Fig. 1(b)], whereas when the pair of disclination lines lies in the microscope field of view, a teardrop pattern can be detected [Fig. 1(c)]. Under homeotropic anchoring and for small relative chirality parameters, an unwound radial profile can be observed,39–41 featuring a central point defect (hedgehog). A twisting deformation around the hedgehog might occur [Fig. 1(e)], as it is energetically less costly than the splay deformation (which exists in a radial configuration). As the chirality parameter increases, the radial structure starts twisting, and the point defect is expelled toward the surface.41 Other common configurations are the so-called nested cup structures39,42,43 [Figs. 1(f) and 1(g)]. There, the isocline planes, that is the planes that share the same director tilt, experience a deformation relative to the droplet curvature. This way they resemble a stack of nested cups. In the case of strong deformation, the cups become almost “closed,” and a disclination line emerges, which does not reach the center of the droplet.36 Overall, we need to point out that perpendicular anchoring conditions result in a frustrating environment for the CLC as they compete with the tendency of the material to form a layer-like helical structure.44 

FIG. 1.

Schematic of common CLC droplet configurations. In (d), (f), and (g), the lines represent the cholesteric layers. (a) Twisted bipolar structure, (b) Frank–Pryce droplet observed in POM along the intertwined pair of disclination lines, (c) POM micrograph of a Frank–Pryce droplet where the pair of disclination lines lies in the microscope field of view, (d) Frank–Pryce structure, (e) radial droplet structure with a twist in the central point defect, and (f) and (g) nested cup structures. There, the cholesteric layers experience a deformation, which is related to the curvature of the droplet, increasingly aligning with the droplet interface. When some of the layers “close,” a radial disclination arises, which does not reach the droplet center.

FIG. 1.

Schematic of common CLC droplet configurations. In (d), (f), and (g), the lines represent the cholesteric layers. (a) Twisted bipolar structure, (b) Frank–Pryce droplet observed in POM along the intertwined pair of disclination lines, (c) POM micrograph of a Frank–Pryce droplet where the pair of disclination lines lies in the microscope field of view, (d) Frank–Pryce structure, (e) radial droplet structure with a twist in the central point defect, and (f) and (g) nested cup structures. There, the cholesteric layers experience a deformation, which is related to the curvature of the droplet, increasingly aligning with the droplet interface. When some of the layers “close,” a radial disclination arises, which does not reach the droplet center.

Close modal

In this context, deciphering CLCs droplet defect topologies is not an easy task, especially as they can have an important role in applications. Here, we are interested in CLC droplets as they are prospective key elements in a volatile organic compounds (VOCs) gas sensing platform. Unlike nematic or smectic-based systems, the sensing potential of CLC droplets is less studied.22,26,36,45 Our approach utilizes a hybrid liquid crystal-based gel formulation we have recently proposed, in order to produce polydisperse self-assembled ionic liquid/liquid crystal droplets embedded in a gelatin matrix. The gel formulation can be easily tuned yielding unique material properties and has been previously explored as a chemical15,16,46,47 or humidity48 sensing composition using nematic and smectic liquid crystal components, featuring radial droplets and rings of toric defects, respectively.15 Since the interplay between spherical confinement and chirality can lead to a great level of topological diversity, we were interested in investigating this approach. We prepared two CLC mixtures with different helical pitches by adding a small fraction of the chiral dopant CB15 [(S)-4-cyano-4′-(2-methylbutyl) biphenyl] to the nematic 5CB (4-cyano-4′-pentylbiphenyl). Considering the polydispersity of the hybrid formulation, a great variety of self-assembled CLC droplet structures was obtained. We first explore the zoology of the droplet topological states generated via spherical confinement in the hybrid gel formulation. Polydispersity is a key aspect in this study as it yields different topologies and optical textures. We then proceed to investigate their optical response in the presence of analyte vapors by means of POM. Overall, the system under study constitutes an effective and simple liquid crystal-based platform to report VOC-recognition events by optical means.49 

The imidazolium-based ionic liquid 1-butyl-3-methylimidazolium dicyanamide ([BMIM][DCA]) was acquired from Iolitec (Heilbronn, Germany, purity > 98%). The liquid crystal 4-cyano-4′-pentylbiphenyl (5CB, purity > 98%) and the chiral dopant (S)-4-cyano-4′-(2-methylbutyl)biphenyl (CB15, purity > 98%) were acquired from TCI Europe. Gelatin from bovine skin (gel strength ≈225 g; Bloom, type B) was supplied by Sigma-Aldrich. Ethanol (purity ≥ 99.8%) was purchased from Sigma-Aldrich. Chloroform (purity > 99.8%), diethyl ether (HPLC grade, purity > 99%), heptane (purity > 99%), and toluene (purity > 99%) were supplied by Fisher Scientific. Acetone (purity ≥ 99.5%) was purchased from Honeywell. Milli-Q water was used. Solvents were of analytical grade and used as received.

The liquid crystal mixtures M1 (3.5% w/w CB15–96.5% w/w 5CB) and M2 (5% w/w CB15–95% w/w 5CB) were prepared by co-dissolving pre-weighted amounts of the individual components in chloroform and allowing the solvent to slowly evaporate at room temperature.

The polydisperse hybrid gel formulations were developed using a magnetic stirrer, through gelation of viscous solutions containing ionic liquid (30–70% w/w), liquid crystal (2–10% w/w), gelatin (15–25% w/w), and Milli-Q water (17–30% w/w), according to the protocol previously reported.15,16,46,47 The final gel composition was deposited onto an untreated glass slide and spread into a film, using an automatic film applicator (TQC Sheen) equipped with a heated bed and a quadruplex with a predefined thickness of 30 and 60 μm. The films were left at room temperature for 24 h before being used.

All the studied gels were characterized by means of Polarizing Optical Microscopy (POM) using a Zeiss Axio Observer.Z1 microscope equipped with an Axiocam 503 color camera and a Zeiss Axioskop 40 microscope with an Axiocam 503 color camera. In both cases, photographs taken were processed by the ZEN 203 software.

The analyte effect on the gels was investigated by means of POM. The films were placed in a custom-made hermetic and transparent glass chamber fixed between the polarizers of the Axio Observer.Z1 microscope. The films were exposed to vapors of six different solvents—heptane, toluene, chloroform, diethyl ether, acetone, ethanol (10 ml of the solvent preheated at 37 °C in a water bath for 15 min in a sample vial to ensure headspace saturation)—for four consecutive cycles. Each cycle consists of a 5 s exposure to gas using an air pump, followed by a 15 s recovery period with ambient air via a second air pump (unless stated otherwise). The two pumps work alternately. The exposure pump carries the VOC vapors from the headspace to the glass chamber. The recovery pump carries ambient air through the system and expels the VOC vapors from the glass chamber and related tubing. The gas flow rate is 50 ml/s. A relay switch unit controls when and which pump should be powered according to which pump was previously activated, thus generating the VOC exposure/recovery cycles. During each VOC experiment, images and videos were recorded using the ZEN software. Frame analysis on the recorded videos was performed using the software ImageJ. The brightness of the video frames along time was determined, representing the light intensity variation transmitted by the gels. The results were plotted against time.

In our previous works,15,16,46–48 we proposed a multicomponent gel composition emerging from the cooperative supramolecular assembly of ionic/liquid crystal droplets supported in a gelatin matrix. It is considered a new class of materials, promising for gas sensing purposes as it combines different component sensing properties in a single material. Physical compartmentalization of the constituents is achieved as the liquid crystal component is encapsulated by an ionic liquid stable interface, whereas the gelatin acts as an immobilization matrix for the self-assembled droplets. The ionic liquid, here 1-butyl-3-methylimidazolium dicyanamide or [BMIM][DCA], is a key component in the formulation as it promotes a preferred orientation on the liquid crystal, and it dissolves the gelatin and further yields a stable and robust gel structure as it does not evaporate due to its low vapor pressure.48 The liquid crystal serves as an optical probe, responding to analyte sorption to the material. An analyte can prompt a decrease in the clearing temperature of the liquid crystal to such an extent that it can potentially trigger a transition. This process is reversible and causes variations in the gel's brightness, setting the basis for optical sensing.

In this work, we produced for the first time chiral liquid crystal hybrid gel materials, which we characterized. The nematic host 5CB (crystal 4-cyano-4′-pentylbiphenyl) was doped with the chiral agent CB15 [(S)-4-cyano-4′-(2-methylbutyl)biphenyl], and two mixtures were made with different compositions: M1 (3.5% w/w CB15–96.5% w/w 5CB) and M2 (5% w/w CB15–95% w/w 5CB). Hybrid gel formulations were prepared using the mixtures M1 and M2 and then spread as thin films on untreated glasses for POM observations. Since both mixtures exhibit a helical periodicity in the micrometer range, they can be both visualized by POM means. The pitches in M1 and M2 materials were measured via the droplet method,50 yielding pitch values of 4.7 and 3.3 μm, respectively. Both gel formulations featured polydisperse CLC droplets with various structures (Fig. 2), and, thus, we decided to investigate the most common droplet configurations observed.

FIG. 2.

POM morphologies under crossed polarizers (a) and (b) and in bright field (c) and (d) of the hybrid gel formulations containing gelatin [BMIM][DCA], and (a) and (c) mixture M1 (3.5% w/w CB15–96.5% w/w 5CB) and (b) and (d) mixture M2 (5% w/w CB15–95% w/w 5CB).

FIG. 2.

POM morphologies under crossed polarizers (a) and (b) and in bright field (c) and (d) of the hybrid gel formulations containing gelatin [BMIM][DCA], and (a) and (c) mixture M1 (3.5% w/w CB15–96.5% w/w 5CB) and (b) and (d) mixture M2 (5% w/w CB15–95% w/w 5CB).

Close modal

Droplets were analyzed by means of POM with crossed polarizers and in bright field and were categorized according to their size and texture. Based on our droplet studies of both hybrid gel formulations and from our previous investigations,15,16 we hypothesize that for small droplets, the ionic liquid [BMIM][DCA] provides an almost homeotropic orientation to the liquid crystal molecules at the droplet interface; however, as the droplet size increases, the orientation becomes degenerate planar. Figure 3 summarizes our findings, where we introduce the dimensionless relative chirality parameter N = 2d/p to facilitate droplet categorization. The droplets are presented with increasing diameter, thus leading to increasing N values. We need to note that the studied droplet configurations are not restricted to a given N value, rather a range of N values, which, in some cases, might exhibit some overlap.

FIG. 3.

Droplet textures with increasing diameter observed under POM with crossed polarizers and in bright field for (a) and (c) M1 (3.5% w/w CB15–96.5% w/w 5CB) hybrid gel and (b) and (d) M2 (5% w/w CB15–95% w/w 5CB) hybrid gel. Black lines correspond to 50 μm.

FIG. 3.

Droplet textures with increasing diameter observed under POM with crossed polarizers and in bright field for (a) and (c) M1 (3.5% w/w CB15–96.5% w/w 5CB) hybrid gel and (b) and (d) M2 (5% w/w CB15–95% w/w 5CB) hybrid gel. Black lines correspond to 50 μm.

Close modal

In the lowest chirality regime, the M1 hybrid formulation (5.7 < N < 8.3) features droplets with a radial structure and a central point defect observed in bright field imaging [Fig. 3(a-1)]. A twist is also detected, typically observed in almost all achiral nematic radial droplets due to a high energetic cost of the increased splay deformation around the hedgehog. Chirality enhances the twist presence.41 As N increases, the next group of droplets commonly encountered (5.3 < N < 11.9) shows a texture that is associated with planar anchoring, whereas in bright field, it exhibits in an almost circular concentric disclination line [Fig. 3(a-2)]. The droplet size is still small, but it allows the phase to start forming layers within the droplets, which are forced to adopt an almost concentric arrangement.42 This droplet configuration could be considered the precursor of the next configuration observed at higher N values. The last droplet configuration found for 9.8 < N < 29.3 exhibits a spiral pattern, which is consistent with a radial arrangement of the CLC helixes [Fig. 3(a-3)], resulting in a texture of equidistant concentric rings with an inter-ring periodicity equal to p/2. It is reminiscent of the Frank–Pryce texture, even though no disclination line could be observed. We termed these droplets as beginning Frank–Pryce droplets. In certain cases, the droplets exhibit a tear drop shape (see Fig. 2). A similar droplet pattern has been detected in multicompartment emulsions of fluorocarbon oils-in CLC-in PVA aqueous solution and also Janus droplets.26 Finally, an “onset” spiral forming droplet texture where the layers start developing spirals (9.8 < N < 22.8) can be seen in Fig. 3(c).

The M2 hybrid gel formulation features radial droplet configurations with a twist originating from the core defect (7 < N < 9.7) as seen in the POM photos in Fig. 3(b-4) taken with crossed polarizers and in bright field. In the range 5.3 < N < 11.9, a droplet texture detected only for the M2 hybrid gel formulation and associated with planar anchoring emerges, as seen in Fig. 3(b-5). It is termed bent-twisted bipolar structure,23,35 and it features two non-antipodal surface defects where their corresponding radii form an angle between 50° and 180°. It is considered an intermediate structure between the twisted bipolar and the Frank–Pryce structure. The next droplet pattern is found in 15.3 < N < 28.4, seen in Fig. 3(b-6), and it exhibits similarities with the droplet in Fig. 3(a-2). It features an almost circular concentric disclination line, indicative of the layer forming propensity of the chiral phase. The last and most common droplet configuration in the M2 hybrid gel is the beginning Frank–Pryce droplet, related to a radial distribution of the chiral helices within the droplet (17.4 < N < 43) seen in Fig. 3(b-7). Finally, similar to the M1 droplets featured in Fig. 3(c), an onset spiraling forming droplet configuration is also found for the M2 droplets, seen in Fig. 3(d), for 17.2 < N < 28.4. Figures 4(a) and 4(b) help in the visualization of the relative chirality ranges related to each droplet configuration and how they might overlap in certain cases. It can be seen [Fig. 4(c)] that the mixture with the higher chirality (i.e., smaller pitch) yields a steeper slope in agreement with previous observations.33 

FIG. 4.

Summary of the droplet configurations investigated in (a) M1 (3.5% w/w CB15–96.5% w/w 5CB) and (b) M2 (5% w/w CB15–95% w/w 5CB) hybrid gels depicted as a plot of the relative chirality against the droplet diameter, and (c) droplet configurations for both hybrid gels are depicted for comparison purposes.

FIG. 4.

Summary of the droplet configurations investigated in (a) M1 (3.5% w/w CB15–96.5% w/w 5CB) and (b) M2 (5% w/w CB15–95% w/w 5CB) hybrid gels depicted as a plot of the relative chirality against the droplet diameter, and (c) droplet configurations for both hybrid gels are depicted for comparison purposes.

Close modal
FIG. 5.

Textural changes of the hybrid gels during exposure to vapors of (a) chloroform and (b) ethanol and subsequent recovery, as observed through the POM and (c) and (d) corresponding signals extracted from video analysis. POM photos were taken with crossed polarizers at room temperature: (a) M1 (3.5% w/w CB15–96.5% w/w 5CB) hybrid gel and (b) M2 (5% w/w CB15–95% w/w 5CB) hybrid gel. The white line corresponds to 50 μm.

FIG. 5.

Textural changes of the hybrid gels during exposure to vapors of (a) chloroform and (b) ethanol and subsequent recovery, as observed through the POM and (c) and (d) corresponding signals extracted from video analysis. POM photos were taken with crossed polarizers at room temperature: (a) M1 (3.5% w/w CB15–96.5% w/w 5CB) hybrid gel and (b) M2 (5% w/w CB15–95% w/w 5CB) hybrid gel. The white line corresponds to 50 μm.

Close modal
FIG. 6.

Signals obtained through video frame analysis using ImageJ from analyte exposure experiments conducted under the polarizing optical microscope for the (a) M1 hybrid gel (5 s exposure/15 s recovery) and (b) M2 hybrid gel (10 s exposure/15 s recovery). Exposure to vapors of heptane, toluene, chloroform, diethyl ether, acetone, and ethanol is portrayed for the duration of four consecutive cycles. Here, an increase in the optical response corresponds to a decrease in the light intensity seen from the microscope.

FIG. 6.

Signals obtained through video frame analysis using ImageJ from analyte exposure experiments conducted under the polarizing optical microscope for the (a) M1 hybrid gel (5 s exposure/15 s recovery) and (b) M2 hybrid gel (10 s exposure/15 s recovery). Exposure to vapors of heptane, toluene, chloroform, diethyl ether, acetone, and ethanol is portrayed for the duration of four consecutive cycles. Here, an increase in the optical response corresponds to a decrease in the light intensity seen from the microscope.

Close modal

Having characterized the droplet configurations for both hybrid gel formulations, we proceeded to test their potential gas sensing abilities. In our previous works,15,16,46,48 we have discussed that when analyte molecules diffuse into the liquid crystal droplets, they can lower the liquid crystal clearing temperature (impurity effect) and, thus, decrease its order parameter to such an extent that a phase transition might be prompted. The analyte-induced phase transitions share optical traits with the thermally induced transitions.15 This is a reversible process upon flushing out the analyte with ambient air. Here, we conducted optical investigations of the VOC effect on the gel compositions, using an in-house built glass chamber fixed between the crossed polarizers of the POM (see Sec. II for details). The tested analytes were heptane, toluene, chloroform, diethyl ether, acetone, and ethanol. Both M1 and M2 hybrid formulations responded to all the tested VOCs (Figs. 5 and 6). During the studies, images and videos were captured for subsequent analysis.

A first example is depicted in Fig. 5(a), which is the case of the M1 hybrid gel exposed to the vapors of chloroform for a single cycle (5 s exposure/15 s recovery). In the initial state, two droplets exhibiting a beginning Frank–Pryce texture are shown. Upon exposure to chloroform, the texture starts to shrink rather quickly [see, for example, in Fig. 5(a) at 0.26 s], however, retaining the spiraling pattern, until the diffusant fully reduces the liquid crystal order and complete isotropization is achieved [see Fig. 5(a) at 0.85 s]. During the recovery period, the analyte is expelled from the periphery of the droplets with ambient air, causing the appearance of bright rings as the isotropic to the chiral nematic phase transition starts [see Fig. 5(a) at 11.46 s]. As the recovery period continues and the phase transition proceeds, the droplets adopt a different and more complex knotted texture with defect lines [see Fig. 5(a) at 13.3 s and at 13.45 s]. This new texture becomes the dominant adopted configuration in the subsequent exposure periods of the experiment. In fact, this observation was confirmed by all analyte exposure experiments. Larger droplets (roughly >40 μm) did not fully recover their initial configuration; however, this is not the case for most smaller droplets as will be discussed in the following example.

The case of the M2 hybrid gel when exposed to ethanol (10 s exposure/15 s recovery) is presented in Fig. 5(b), where we monitor droplets exhibiting the onset spiraling texture [seen in Fig. 3(c) and 3(d)]. Ethanol absorbance into the droplets during exposure significantly reduces the liquid crystal order, triggering a chiral nematic to isotropic transition [see, for example, in Fig. 5(b) at 1.17 s]. In this case, however, the phase transition is not concluded during exposure [Fig. 5(b) at 9.83 s]. In the recovery period, where the ethanol vapors are flushed out of the system, the reverse process is observed, and the liquid crystal droplets almost fully recuperate their initial configuration, as seen in Fig. 5(b) at 21.94 s.

The videos recorded during our investigations were treated with the ImageJ software. With frame analysis, we were able to assess the brightness of the frames, which can be considered a measure of the light intensity variation transmitted by the liquid crystal formulations. The results were plotted against time, as seen in Figs. 5(c) and 5(d), and can offer a preliminary estimation of the gels performance. An increase in the optical response in the plots corresponds to a decrease in the light intensity seen from the microscope. The overall variations in the waveforms reflect the dynamic disorganization and reorganization of the droplets. As seen in Figs. 5(c) and 5(d), the two gel formulations exhibit quite different response profiles. M1 hybrid gel responds quickly to the chloroform vapors, and complete isotropization (the plateau in the response profile) is achieved within 1.8 s of exposure. On the other hand, the recovery process is rather slow, since it starts after approximately 6.5 s within the recovery period. We also need to point out that overall, the optical profile retains its shape during the experiment, and it is reproducible. A similar waveform pattern was obtained from the M2 hybrid formulation when exposed to chloroform (see Fig. 6). In this case, however, the gel responded faster, and a signal plateau (full isotropization) was observed within 0.5 s. Moreover, the M2 gel compound recovered faster than the corresponding M1, as the process starts roughly 2.4 s within the recovery period.

The signal pattern exhibited by the M2 hybrid gel for ethanol exposure is very different. The signal never reaches a plateau as complete liquid crystal isotropization was never achieved, and the recovery process takes place as soon as the diffusant starts to be expelled from the system. Also, the signal shape is not fully retained during the experiment, which is related to the dynamics taking place between the formulation and the diffusant. We need to point out that a similar response profile has been observed for the M1 formulation (see Fig. 6). Overall, the two hybrid formulations exhibited similar waveform patterns for the majority of the tested VOCs, though it was observed that the M1 hybrid gel was slower in its response and recuperation processes. One can hypothesize that increasing the chiral agent CB15 content in the chiral mixtures enhances the liquid crystal sensitivity toward the tested analytes.

Based on our previous investigations, the signal profiles are the results of the collaborative response of the distinct compartments of the gel toward the corresponding analyte.15,16,46,48 Thus, waveform shapes and response times mirror the potential analyte affinity toward specific compartments, whereas the recovery times are mostly influenced by the geometry of the system and the subsequent restrictions imposed on the gel components.15 With regard to analyte affinity, we have already established that apolar hydrophobic analytes (such as heptane and toluene) tend to interact preferentially with the liquid crystal component of the formulation, by lowering its isotropization temperature. On the other hand, protic and hydrogen bond forming analytes (such as ethanol) may show a proclivity to the gelatin and ionic liquid moieties. This can result in the disruption of the ionic liquid interfaces, indirectly disorganizing the liquid crystal, and may lead to potential droplet mobility within the gelatin matrix. For analytes with intermediate polarities and hydrogen bonding capabilities, we believe there are combined mechanisms of interaction related to the degree of predominance toward the different components of the hybrid formulation. Hence, different analyte proclivities may lead to different magnitudes of response within different time frames and could affect the dynamic disorganization and reorganization processes of the liquid crystal in a unique way. Here, for example, it is likely that chloroform interacts preferentially with the liquid crystal component in the M1 hybrid gel (see video S1). This was also observed for the M2 formulation. However, when testing the M2 gel with ethanol, the analyte showed a tendency toward the gelatin and ionic liquid components, as was the case for the M1 hybrid gel. The ionic liquid interface for many droplets was disrupted, and droplet mobility was observed, which led to subsequent droplet merging. All these effects result in a gradual accumulation of permanent changes in the morphology of the gel and the liquid crystal organization with time, leading to less stable signal responses as it was seen in Fig. 5(d). More details on this can be found in video S2.

We also performed preliminary experiments on an optical electronic nose device developed by our group, followed by signal processing and analysis similar to the works of Ramou et al.15 and Esteves et al.16 (see the supplementary material). It was found that the two chiral gel formulations exhibited different accuracies of VOC prediction, both when compared to the purely nematic 5CB-based formulation15 and to each other. For example, the M1 formulation was the most successful in the discrimination of chloroform vapors, whereas the M2 hybrid gel yielded the best classification results for diethyl ether and toluene, and the nematic 5CB-based composition performed most accurately under the presence of acetone. On the other hand, all three formulations gave comparable results for ethanol and heptane. These preliminary findings altogether truly support the notion that an array of sensors is the most effective approach for artificial olfaction purposes.

From a structural point of view, the diversity exhibited by the polydisperse gel systems renders them very fascinating media for the study of such complex architectures in pursuit of not only understanding the mechanisms governing their organization but also developing functional programmable materials. From a technological point of view, the extreme sensitivity to analyte vapors exhibited by the CLC-based gels is a very desirable feature for designing small, autonomous, and sustainable sensor devices with low energy consumption. Further improvement to the potential of the CLC-based gel could be introduced by enhancing the selectivity of the response. This could be achieved by the addition of chemical moieties with affinities toward specific analytes or by exploring different ionic liquids or even chiral additives. It could also be interesting to study mixtures of higher chirality with a pitch comparable to the wavelength of visible light. In this instance, visible light is selectively reflected by the chiral liquid crystal phase, and the system becomes iridescent, i.e., any occurring changes can be observed with the naked eye. We believe that further optimization of the system in line with these approaches could be a promising route toward alternative, efficient, and robust stimuli-responsive platforms.

To summarize our findings, we report self-assembled soft materials where chiral liquid crystal formulations are confined as droplets by an interface formed by ionic liquids and further encapsulated within a gelatin matrix. We investigated two chiral liquid crystal-based gel formulations featuring polydisperse self-assembled droplets with a wealth amount of different topological architectures and exhibiting sensitivity to analyte vapors. We studied and categorized the various droplet morphologies with regard to their relative chirality parameter. Among the various defect structures that we unveiled, we found that both CLC-based gels exhibit in the lowest chirality regime a twisted radial structure with a central point defect, whereas a beginning Frank–Pryce texture—a spiral pattern consistent with a radial arrangement of the CLC helices—is found in the highest chirality regime. Additionally, only for the M2 hybrid gel formulation, we observed a bent-twisted bipolar structure in the range 5.3 < N < 11.9. We also tested their responsive capabilities to the presence of VOC analytes, while observing in real time through POM the occurring textural transformations unique for each VOC tested. The CLC-based materials respond in a dynamic alternation between the chiral nematic and the isotropic phases. The complexity and richness emerging from the observed morphological changes could be advantageous, when coupled with image recognition tools, for automated report and identification of gaseous analytes and further provide important insights toward the development of small yet versatile sensing platforms. From an application perspective, the newly developed chiral liquid crystal materials can be used as semi-selective sensors in arrays of gas sensors used in electronic nose devices.

See the supplementary material for more details on (i) the M1-based gel when exposed to chloroform vapors and the M2-based gel when exposed to ethanol vapors in the form of video and (ii) preliminary optical e-nose experiments and associated confusion matrices.

This project has received funding from the European Research Council (ERC) under the EU Horizon 2020 research and innovation programme (SCENT-ERC-2014-STG-639123, 2015–2022 and Grant Agreement No. 101069405—ENSURE—ERC-2022-POC1) and by national funds from FCT-Fundação para a Ciência e a Tecnologia, I.P., for projects PTDC/BII-BIO/28878/2017, PTDC/CTM-CTM/3389/2021, UIDP/04378/2020, and UIDB/04378/2020 of the Research Unit on Applied Molecular Biosciences-UCIBIO and the project LA/P/0140/2020 of the Associate Laboratory Institute for Health and Bioeconomy-i4HB.

The authors have no conflicts to disclose.

Efthymia Ramou: Conceptualization (supporting); Data curation (lead); Formal analysis (lead); Investigation (lead); Validation (lead); Writing – original draft (lead); Writing – review & editing (equal). Ana Cecilia Afonso Roque: Conceptualization (lead); Funding acquisition (lead); Investigation (supporting); Project administration (lead); Resources (lead); Supervision (lead); Validation (equal); Writing – review & editing (equal).

The data that support the findings of this study are available from the corresponding authors upon reasonable request.

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Supplementary Material