Surface acoustic wave sensing of linear alcohols using para-acylcalix[n]arenes

Abstract

para-Hexanoylcalix[4, 6 or 8]arenes have been used as surface acoustic wave sensor capture layers showing a high efficiency for the detection of linear alcohols, with high reproducibility and rapid response times.

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Alcohol sensing is an evidently important subject especially with regard to ethanol detection in human subjects.¹ However the detection of other volatile alcohols has received far less attention.² Methods for detection of alcohols range from colorimetric,³ conductimetric⁴ to quartz crystal microbalance (QCM).⁵ Other highly sensitive gravimetric measurements such as surface acoustic wave sensors (SAW)⁶ would seem to be prime targets for expanding methods for the detection of volatile organic molecules (VOCs).

The sensitivity of such devices is very much determined by the nature of the active capture on the device surface and probably also by the nature of the layer deposition method. Evidently supramolecules such as cyclodextrins,⁷ cryptophanes,⁸ and calix[n]arenes⁹ are targets for use as the sensing layers in VOC sensors.

The calix[n]arenes are highly attractive to use as the sensing layers as they are readily available,¹⁰ and are relatively simple to modify¹¹ thus allowing the use of a wide range of molecules having different sizes and complexation properties. This choice is reinforced by the known capabilities of the calix[n]arenes to act as porous systems even when structurally non-porous as elegantly described by Barbour et al.¹² Their versatility for gas capture has been shown by Ripmeester,¹³ Rudkevich,¹⁴ Cao,¹⁵ Davis,¹⁶ and others.

In this communication we have investigated the capture of a series of linear alcohols, with chain lengths between one and twelve carbon atoms by SAW sensors using para-acylcalix[n]arenes as the sensing layers. We have further investigated the morphology of the layers using atomic force microscopy (AFM), here there are clear differences between the structures of the thin films.

The molecular structures of the para-acylcalix[n]arenes are shown is Scheme 1, the acyl chain length was retained as six carbon atoms to limit the number of variables. The experimental setup consists of a chamber of approximately 2 litres in which a two channel chip is inserted. (One channel is not functionalised and on the other the thin film of para-acylcalix[n]arene is deposited.) The chamber can alternately be put under vacuum or subjected to the alcohol vapour. All the experiments were undertaken at room temperature. The results are highly reproducible either for a single deposit or between different thin films.

Scheme 1 para-Hexanoylcalix[4]arene (a) C6C4, para-hexanoylcalix[6]arene (b) C6C6 and para-hexanoylcalix[8]arene (c) C6C8.

A typical sensogram is shown in Fig. 1, all others are given in the ESI†, for the capture of ethanol by para-hexanoylcalix[n]arenes. The reproducibility of frequency change and sensing curve are evident. It can be seen that the adsorption stage and the desorption stage are not symmetrical implying that different forces are exerted on molecules entering or leaving the host matrix. For adsorption the main uptake occurs within 10 seconds and there is a slow equilibration over 5 minutes, this is reversed for desorption with a rapid initial desorption again in a few seconds followed by slow equilibration, a typical cycle can be achieved in about 10 minutes. There is, however, a great deal of variation in the sensogram forms with implications for both the kinetics of adsorption–desorption and intermolecular interactions between the para-acylcalix[n]arene host matrix and the alcohols.

Fig. 1 Response curve of a SAW coated with para-hexanoylcalix[4]arene toward ethanol.

The adsorption behaviour can be measured by plotting the average changes in frequency against a number of variables, as SAW sensors are gravimetric in nature one likely variable is the molecular mass of the adsorbed molecule, which is given in Fig. S1 (ESI†). In general there is poor correlation between sensor response and the molecular weight of the analyte.

Another possible physical property that might correlate with the adsorption behaviour is the vapour pressure of the alcohol as shown in Fig. 2.

Fig. 2 Representation of Δf against alcohol vapour pressure of methanol (1), ethanol (2), propan-1-ol (3), butan-1-ol (4), pentan-1-ol (5), hexan-1-ol (6), octan-1-ol and decan-1-ol (8) for para-hexanoylcalix[4]arene (a), para-hexanoylcalix[6]arene (b) and para-hexanoylcalix[8]arene (c). Vapour pressure data from ChemSpider RSC.

Here the correlation is improved, for para-hexanoylcalix[4]arene a straight line can be plotted, with y = 7.9x + 170. For para-hexanoylcalix[6]arene the situation is quite different and the curve can best be described as an exponential, y = 0.01x³ + 0.12x² + 10.7x + 110. For para-hexanoylcalix[8]arene a straight line may be plotted, y = 5.6x + 195. If the controlling factor was simply the vapour pressure all these plots should be linear with identical slopes, this is clearly not the case and other factors should be considered.

At low pressures there is a large scatter for the points and interestingly it can be noted that pentanol always shows a higher frequency change when compared to butanol.

Obviously a major concern in sensor development is the capacity to discriminate between chemical entities, in Table 1 we give the frequency changes for the three capture molecules sensing octanol and its aldehyde analogue octanal. The vapour pressure of octanal is about 15 times that of octanol, and thus we should see a higher frequency change for octanal. Only for para-hexanoylcalix[6]arene this is the case. para-Hexanoylcalix[8]arene does not discriminate between the molecules and para-hexanoylcalix[4]arene shows a higher frequency change for the less volatile molecule. The use of the three molecules permits a clear discrimination between the two structurally very similar analytes.

Table 1 Comparison of frequency responses between octanol and octanal

Compound	Vapor pressure/mm Hg	Δf C6C4	Δf C6C6	Δf C6C8
Octanol	0.14	150	215	55
Octanal	2	90	600	55

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Deposition of the same solutions and the same quantities of material onto gold surfaces and followed by AFM analysis gives a probable explanation for the observed behaviour. AFM images at the 5 × 5 μm scan range are shown below for the three para-hexanoylcalix[n]arenes (Fig. 3).

Fig. 3 Atomic force microscopy images of para-hexanoyl[4]arene (a), para-hexanoyl[6]arene (b) and para-hexanoyl[8]arene surfaces deposited on gold. Image sizes are 5 × 5 μm.

The surfaces show clear visible differences with those of para-hexanoylcalix[4]arene and para-hexanoylcalix[8]arene showing relatively flat surfaces composed of cavities whereas the surface of para-hexanoylcalix[6]arene appears to be composed of micro-crystallites. This structuration is reflected in the measurement of the surface roughness (RMS, root mean square surface roughness average), with values of 0.7 nm for the calix[4]arene, 28.9 nm for the calix[6]arene and 3.5 nm for the calix[8]arene. Such an increased roughness is a direct reflection of the presence of crystallites. For para-hexanoylcalix[4]arene and para-hexanoylcalix[8]arene only a single factor is involved in the uptake, that is diffusion into the layer with rapid access to the capture molecule, however for para-hexanoylcalix[6]arene two factors would seem to be present in the equation describing the correlation between sensor response and the vapour pressure, these would be diffusion into the crystallites and then into the capture molecules.

In conclusion we have demonstrated that SAW sensors based on para-hexanoylcalix[n]arene capture layers are highly efficient for the detection of linear alcohols, with high reproducibility and rapid response times. The detection behaviour is best described in terms of the analyte vapour pressure and is related to the structuring of the active layer on the transducer surface.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c1cc15352h

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