Technical Field

[001] The invention relates to volatile organic compound vapour sensing compounds. In particular, present invention relates to said sensing compounds comprising a metal phthalocycanine. Furthermore, the invention relates to the tuning sensitivity of the metal phthalocyanine by incorporation of substitution groups.

Background

[002] Volatile organic compound (VOC) vapours are primary sources of indoor environmental pollutants and are harmful to the human and animal body. There are, notable correlations between VOC emissions and different kinds of cancers. Therefore, more work is needed concerning VOC vapours sensing and sensor development. Furthermore, indoor air quality information can be given as C0 ₂ equivalents by measuring total VOC concentrations.

[003] Existing VOC vapour sensing inorganic materials include surfactant-coated metal alloy nanoparticles, coated single-walled carbon nanotubes, metal oxide semiconductors such as zinc oxide (ZnO), tin dioxide (Sn0 ₂ ), tungsten trioxide (W0 ₃ ), and titanium dioxide (Ti0 ₂ ). However, such inorganic sensing materials have shortcomings including high energy consumption, low selectivity and sensitivity.

[004] Existing VOC vapour sensing organic materials include compounds comprising a polythiophene, a polypyrrole, and a polyaniline. However, the unsatisfying sensitivity, selectivity, processability, and life time impede them as suitable candidates as VOC vapour sensing materials.

[005] Therefore, there remains a need to provide for alternative VOC vapour sensing compounds that overcome, or at least alleviate, the above problems. Summary

[006] Present inventors have herein identified phthalocyanine, a versatile aromatic macrocycle, as a promising VOC vapour sensing compound. Compared to other organic macrocyclic compounds, phthalocyanine has good processability, stability, tunability, and high selectivity achievable by tuning its central cavity and substitution groups.

[007] Thus, in accordance with one aspect of the invention, there is provided a use of a compound of Formula (I)

as a volatile organic compound vapour sensor, wherein: ^"

M is selected from the group consisting of Mn ²⁺ , Mg ²⁺ , Ca ²⁺ , B ³⁺ , Fe ³⁺ , Al ³⁺ , Ga ³⁺ , ln ³⁺ , Ce ³⁺ , Sc ³⁺ , Zr ⁺ , Ti ⁴⁺ , Sn ⁴⁺ , and V ⁵⁺ ; and each of R ¹ to R ¹⁶ is independently a H or an electron withdrawing group moiety selected from the group consisting of -N0 ₂ , -NH ₃ ⁺ , -S0 ₃ H, -CN, -C(0)CI, -COOH, -CHO, -F, -CI, -Br, -I, - CF ₃ , and -CCI ₃ , with the provisio that not all of R ¹ to R ¹⁶ are H.

[008] In various embodiments, the compound of Formula (I) may be used to detect volatile organic compounds such as amines, nitrogen-containing organic compounds besides amines, sulfur-containing organic compounds, phosphines, alcohols, carbonyl compounds (including ketones, aldehydes, esters, ethers), water, aromatic compounds, alkenes, alkynes, and preferably acetone.

[009] In various embodiments, use of the compound of Formula (I) may incude use as a sensing layer in various transducers like a chemiresistor, a capacitor, a field effect transistor (FET), an optical-based sensor, or a mass-based sensor.

[010] In another aspect of the invention, a sensor for detecting a volatile organic compound vapour is disclosed. The sensor comprises a compound of Formula (I)

wherein:

M is selected from the group consisting of Mn ²⁺ , g ²⁺ , Ca ²⁺ , B ³ \ Fe ³⁺ , Al ³⁺ , Ga ³⁺ , ln ³⁺ , Ce ³⁺ , Sc ³⁺ , Zr ⁴⁺ , Ti ⁴⁺ , Sn ⁴⁺ , and V ⁵⁺ ; and each of R ¹ to R ¹⁶ is independently a H or an electron withdrawing group moiety selected from the group consisting of -N0 ₂ , -NH ₃ ⁺ , -S0 ₃ H, -CN, -C(0)CI, -COOH, -CHO, -F, - CI, -Br, -I, -CF ₃ , and -CCI _3l with the provisio that not all of R ¹ to R ⁶ are H. Brief Description of the Drawings

[011] In the drawings, like reference characters generally refer to the same parts throughout the different views. The drawings are not necessarily drawn to scale, emphasis instead generally being placed upon illustrating the principles of various embodiments. In the following description, various embodiments of the invention are described with reference to the following drawings.

[012] Figure 1 shows the response of AI(lll)-phthalocyanine and Cu(ll)-phthalocyanine sensing layer to acetone from 0.2 ppm to 3 ppm.

[013] Figure 2 shows the response ^' of AIPc-EWG to acetone at 0.2, 0.5, 1 , 2, and 3 ppm. The results are reproducible for three continuous cycles.

Description

[014] The following detailed description refers to the accompanying drawings that show, by way of illustration, specific details and embodiments in which the invention may be practised. These embodiments are described in sufficient detail to enable those skilled in the art to practise the invention. Other embodiments may be utilized and structural and chemical changes may be made without departing from the scope of the invention. The various embodiments are not necessarily mutually exclusive, as some embodiments can be combined with one or more other embodiments to form new embodiments.

[015] Phthalocyanine demonstrates various advantages over other organic compounds as a VOC sensing material. For example, phthalocyanine has good processability, stability, tunability, and high selectivity achievable by tuning its central cavity and substitution groups.

[016] Accordingly, present inventors have shown that selectivity of the present metal phthalocyanine towards VOC vapour detection can be tuned and therefore enhanced by selectively manipulating the metal centre and substitution groups on the phthalocyanine core structure. Furthermore, adjusting the number of substitution groups on the phthalocyanine core structure also helps to further fine-tune the sensitivity and selectivity.

[017] The interaction, and therefore detection, of a VOC vapour and the present metal phthalocyanine sensing compound is based on Lewis acid/base concept. Briefly, Lewis acid is a species that accepts an electron pair and has vacant orbitals. Lewis base is a species that donates an electron pair and has lone pair of electrons. When a Lewis base donates its lone pair of electrons to a Lewis acid, an acid-base complex or complex ion is formed.

[018] In the present context, the metal Cation in the metal phthalocyanine acts as a Lewis acid since the metal cation has an unfilled valence shell and is thus able to accept electrons from a VOC molecule, such as but not limited to acetone, with a lone pair of electrons on the oxygen atom acting as a Lewis base.

[019] While in theory all metal centres could be treated as a Lewis acid to interact with Lewis base, many VOCs, including acetone, are relatively weak Lewis base and therefore stronger Lewis acids are required for an effective sensing. In this context, present inventors have compared different metal centres to study their strengths as suitable Lewis acids: Al ³⁺ , Fe ³⁺ and Cu ²⁺ . As confirmed by studies (to be described in later paragraphs), the attacking strength of AI(lll)-phthalocyanine and Fe(lll)-phthalocyanine are higher and therefore the sensing response is stronger than that of Cu(ll)-phthalocyanine since Al(lll) and Fe(lll) cations are more positively charged.

[020] Furthermore, according to Hard Soft Acid Base theory, hard acids and bases are small, compact, and non-polarizable. Soft acids and bases are larger, with a more diffuse distribution of electrons. Hard acids react preferentially with hard bases and soft acids react preferentially with soft bases. Acetone, for example, is regarded as a hard Lewis base and prefers to react with a hard Lewis acid. Table 1 lists the hardness degree for the above-mentioned metal centres (to be described in later paragraphs). Based on the hardness degree, it can be readily seen that the acetone prefers to react with AI(lll)-phthalocyanine, Fe(lll)-phthalocyanine, and Cu(ll)-phthalocyanine in descending order.

[021] As mentioned above, in addition to tuning the metal centre of the phthalocyanine core structure, tuning of the substitution groups can also enhance the sensitivity of present sensing compounds to VOC vapour.

[022] In present context, the sensitivity to VOC may be influenced by the acidity of the metal phthalocyanine. The acidity of the Lewis acid, i.e. the metal phthalocyanine, can be tuned by introducing an electron withdrawing group as a substitution group. An electron withdrawing group (EWG) is a functional group that can attract electron density towards itself from a conjugated n system. The electron withdrawing group present in the Lewis acid increases its electron deficiency and thus enhances its electron accepting ability, thereby increasing its interaction strength with a Lewis base.

[023] On the other hand, an electron donating group present in a Lewis acid increases its electron density and thus reduces its electron accepting ability, thereby decreasing its interaction strength with a Lewis base. An electron donating group (EDG) is a functional group that can donate electron density to a conjugated π system. Accordingly, it is the intention of present invention to avoid or minimise the use of an electron donating group as a substitution group in present metal phthalocyanine.

[024] Based on the above design parameters, present invention therefore relates to a use of a compound of Formula (I)

as a volatile organic compound vapour sensor, wherein:

M is selected from the group consisting of Mn ²⁺ , Mg ²⁺ , Ca ²⁺ , B ³⁺ , Fe ³⁺ , Al ³⁺ , Ga ³⁺ , ln ³⁺ , Ce ³⁺ , Sc ³⁺ , Zr ⁺ , Ti ⁺ , Sn ⁴⁺ , and V ⁵⁺ ; and each of R ¹ to R ¹⁶ is independently a H or an electron withdrawing group moiety selected from the group consisting of -N0 ₂ , -NH ₃ ⁺ , -S0 ₃ H, -CN, -C(0)CI, -COOH, -CHO, -F, -CI, -Br, -I, - CF ₃ , and -CCI ₃ , with the provisio that not all of R ¹ to R ¹⁶ are H.

[025] In addition, the above-listed moieties are electron withdrawing relative to H atom.

[026] M in Formula (I) is selected from the group consisting of Mn ²⁺ , Mg ²⁺ , Ca ²⁺ , B ³⁺ , Fe ³⁺ , Al ³⁺ , Ga ³⁺ , ln ³⁺ , Ce ³ \ Sc ³⁺ , Zr ⁺ , Ti ⁴⁺ , Sn ⁺ , and V ⁵⁺ .

[027] For Example, M in Formula (I) may be Al ³⁺ or Fe ³⁺ , preferably Al ³⁺ .

[028] In various embodiments, in Formula (I) at least one of R ¹ to R ¹⁶ is an electron donating group moiety.

[029] In certain embodiments, in Formula (I) at least one of R ¹ to R ¹⁶ is F. For example, at least R ⁴ , R ⁸ , R ¹² , and R ¹⁶ are F. [030] In yet further embodiments, in Formula (I) R ⁴ , R ⁸ , R ¹² , and R ¹⁶ are F and R ¹ , R ² , R ³ , R ⁵ , R ⁶ , R ⁷ , R ⁹ , R ¹⁰ , R ¹¹ , R ¹³ , R ¹⁴ , and R ¹⁵ are H.

[031] In alternative embodiments, in Formula (I) R ⁴ , R ⁸ , R ¹² , and R ¹⁶ are F and each of R ¹ , R ² , R ³ , R ⁵ , R ⁶ , R ⁷ , R ⁹ , R ⁰ , R ¹¹ , R ¹³ , R ¹⁴ , and R ¹⁵ is any of the listed electron withdrawing group moiety other than F.

[032] In one embodiment, in Formula (I) M is Al ³⁺ , R ⁴ , R ⁸ , R ¹² , and R ¹⁶ are F, and R ¹ , R ² , R ³ , R ⁵ , R ⁶ , R ⁷ , R ⁹ , R ¹⁰ , R ¹¹ , R ¹³ , R ¹⁴ , and R ¹⁵ are H.

[033] The VOC vapour sensor of Formula (I) is able to detect VOCs including, but not limited to, amines, nitrogen-containing organic compounds besides amines, sulfur-containing organic compounds, phosphines, alcohols, carbonyl compounds (including ketones, aldehydes, esters, ethers), water, aromatic compounds, alkenes, alkynes, and preferably acetone. In one embodiment, the VOC vapour sensor of Formula (I) detects acetone.

[034] In various embodiments, use of the compound of Formula (I) may incude use as a sensing layer in various transducers like a chemiresistor, a capacitor, a field effect transistor (FET), an optical-based sensor, or a mass-based sensor.

[035] In another aspect of the invention, a sensor for detecting a volatile organic compound vapour is disclosed. The sensor comprises a compound of Formula (I)

(I)

wherein:

M is selected from the group consisting of Mn ²⁺ , Mg ²⁺ , Ca ²⁺ , B ³⁺ , Fe ³⁺ , Al ³⁺ , Ga ³⁺ , ln ³⁺ , Ce ^3t , Sc ³⁺ , Zr ⁴⁺ , Ti ⁴⁺ , Sn ⁴⁺ , and V ⁵⁺ ; and each of R ¹ to R ¹⁶ is independently a H or an electron withdrawing group moiety selected from the group consisting of -N0 ₂ , -NH ₃ ⁺ , -S0 ₃ H, -CN, -C(0)CI, -COOH, -CHO, -F, - CI, -Br, -I, -CF _3l and -CCI ₃ , with the provisio that not all of R ¹ to R ¹⁶ are H.

[036] In order that the invention may be readily understood and put into practical effect, particular embodiments will now be described by way of the following non-limiting examples.

Examples

Tuning sensitivity to VOC by tuning metal centres

[037] Chemiresistors with various metal phthalocyanine layers were prepared by evaporation method, which included AI(lll)-phthalocyanine, Fe(lll)-phthalocyanine, and Cu(ll)- phthalocyanine. The thickness of each sensing layer was 60 nm. The respective current change at applied 5 V when exposed to acetone vapour at various concentrations was evaluated. Cu(ll)-phthalocyanine showed no response to acetone even at the highest evaluated concentration level of 10 ppm. Fe(lll)-phthalocyanine showed better sensitivity than Cu(ll)- phthalocyanine and the lowest detection limit was 1 ppm. AI(lll)-phthalocyanine was the most sensitive layer and was able to detect as low as 0.2 ppm (Figure 1 ).

[038] To better quantify the sensitivity of the respective sensing layer, the sensitivity defined by "% sensitivity/ ppm" was evaluated. First, the % current change (equating to [(/o-/ _f )/ /o]x100) was determined. Then the linear trend was obtained from the calculated % current change at different concentrations, and the slope of the line was regarded as "% sensitivity/ ppm".

[039] The sensitivity results for the various sensing layers are summarized in Table 1. The trend of sensitivity correlates with the hardness of the various metal centres. Additionally, it was also found that work function of the AI(lll)-phthalocyanine layer will change when exposed to acetone vapour at low concentrations.

[040] Table 1. Correlation between hardness of the metal centre and the sensitivity to acetone

Tuning sensitivity to VOC by tuning substitution groups

[041] Chemiresistors with various substituted Al-phthalocyanine layers were prepared, which included AI(lll)-phthalocyanine without substitution ("AIPc"), AI(lll)-phthalocyanine substituted with -S(phenyl) at each of R ⁴ , R ⁸ , R ¹² , and R ¹⁶ ("AIPc-EDG"), and AI(lll)-phthalocyanine substituted with -F at each of R ⁴ , R ⁸ , R ² , and R ¹⁶ ("AIPc-EWG"). The thickness of each sensing layer was 60 nm. The respective current change at applied 5 V when exposed to acetone vapour at 0.2, 0.5, 1 , 2, and 3 ppm were evaluated. [042] AIPc-EDG showed the lowest sensitivity while AIPc-EWG showed the highest sensitivity. The results are summarized in Table 2. Additionally, AIPc-EWG showed good dose- response and reproducibility for acetone sensing at low concentration (Figure 2).

[043] Table 2. Correlation between substitution group and the sensitivity to acetone

[044] By "comprising" it is meant including, but not limited to, whatever follows the word "comprising". Thus, use of the term "comprising" indicates that the listed elements are required or mandatory, but that other elements are optional and may or may not be present.

[045] By "consisting of" is meant including, and limited to, whatever follows the phrase "consisting of". Thus, the phrase "consisting of" indicates that the listed elements are required or mandatory, and that no other elements may be present.

[046] The inventions illustratively described herein may suitably be practiced in the absence of any element or elements, limitation or limitations, not specifically disclosed herein. Thus, for example, the terms "comprising", "including", "containing", etc. shall be read expansively and without limitation. Additionally, the terms and expressions employed herein have been used as terms of description and not of limitation, and there is no intention in the use of such terms and expressions of excluding any equivalents of the features shown and described or portions thereof, but it is recognized that various modifications are possible within the scope of the invention claimed. Thus, it should be understood that although the present invention has been specifically disclosed by preferred embodiments and optional features, modification and variation of the inventions embodied therein herein disclosed may be resorted to by those skilled in the art, and that such modifications and variations are considered to be within the scope of this invention.

[047] By "about" in relation to a given numerical value, such as for temperature and period of time, it is meant to include numerical values within 10% of the specified value.

[048] The invention has been described broadly and generically herein. Each of the narrower species and sub-generic groupings falling within the generic disclosure also form part of the invention. This includes the generic description of the invention with a proviso or negative limitation removing any subject matter from the genus, regardless of whether or not the excised material is specifically recited herein.

[049] Other embodiments are within the following claims and non- limiting examples. In addition, where features or aspects of the invention are described in terms of Markush groups, those skilled in the art will recognize that the invention is also thereby described in terms of any individual member or subgroup of members of the Markush group.