Patent Description:
This invention was made with government support under <NUM>-ST-<NUM>-LR0005 awarded by the U. Department of Homeland Security and grant CHE0931466 and grant CBET1502433 awarded by the National Science Foundation. The government has certain rights in the invention.

This invention relates to the field of vapor or gas detection using compounds based on perylene-tetracarboxylic diimide. Accordingly, the invention involves the fields of organic chemistry, chemical engineering, and nanotechnology.

Peroxide explosives, including TATP, DADP, and HMTD, represent one class of the most elusive explosives that can be easily made at home from commercially available products. The ease of preparation, together with the tremendous explosive power and easy initiation makes peroxide explosives preferred by terrorists and insurgents in making improvised explosive devices (IEDs). IEDs are one of the three major types of explosives of particular interest to the United States Department of Homeland Security. Current technologies cannot detect all required explosives with the speed, specificity, and distance demanded by checkpoint security. Furthermore, current detection systems are expensive. Therefore, it is emergent to develop an inexpensive but efficient method for peroxide explosive detection. H<NUM>O<NUM> is commonly used as the chemical marker of peroxide explosives, which is often leaked from organic peroxides as a synthetic impurity, or can be produced from the chemical decomposition of peroxide explosives. Therefore, a sensory material with high sensitivity and selectivity to H<NUM>O<NUM> would be beneficial for peroxide explosive detection. The development of a low-power sensor device that could provide inexpensive and simple peroxide detection could replace the current expensive explosive detection equipment with dependable and affordable sensors.

Most oxidant gases or vapors are hazardous chemicals, which need to be controlled and monitored. H<NUM>O<NUM> is an industrial chemical widely used in applications such as waste water processing, paper manufacturing, bleaching, toothpaste, and hair color. H<NUM>O<NUM> can be toxic if ingested, inhaled, or by contact with the skin or eyes. Inhalation of household strength H<NUM>O<NUM> (<NUM>%) can cause respiratory irritation. Exposure to household strength H<NUM>O<NUM> can cause mild ocular irritation. NO<NUM> is a well-known oxidant gas, which is produced from fossil fuel combustion processes, and is one of the most dangerous air pollutants. NO<NUM> plays a major role in the formation of ozone and acid rain. Continued or frequent exposure to NO<NUM> at higher than air quality standard may cause increased incidence of acute respiratory illness. Chemiresistive vapor sensing compounds based on perylene-tetracarboxylic diimide (PTCDI) are known, for example, from <CIT>, from <NPL>, from <NPL>, and from Shuai <NPL>.

A high efficiency, small, light, and low-power oxidant vapor sensor for real-time monitoring of hazardous oxidant gas would be advantageous.

A chemiresistive vapor sensor compound for detecting target vapors according to the present invention comprises a perylene-tetracarboxylic diimide (PTCDI) core according to structure (I):
<CHM>
where R is a morphology control group or -A'-D', A and A' are independently a linking group, D and D' are independent strong electron donors which transfer electrons to the PTCDI core of an adjacent molecule of the compound sufficient to form an anionic PTCDI radical of the PTCDI core, with both D and D' creating a ΔG <<NUM> for formation of the anionic PTCDI radical that facilitates the formation of the anionic PTCDI radical without photo-excitation, and R1 to R8 can be independently a side group. are independent side groups or hydrogen.

A chemiresistive vapor sensor for detection of a target compound can comprise an assembly of nanofibers formed of the chemiresistive sensor compound and a pair of electrodes operatively oriented about the assembly of nanofibers to allow electrical current to pass from a first electrode in the pair of electrodes through the assembly of nanofibers and to a second electrode in the pair of electrodes.

In one aspect, a method of detecting target compounds can comprise exposing the assembly of nanofibers to a suspected target compound source, measuring an electrical response of the assembly of nanofibers, and displaying a detection metric based on the electrical response.

There has thus been outlined, rather broadly, the more important features of the invention so that the detailed description thereof that follows may be better understood, and so that the present contribution to the art may be better appreciated. Other features of the present invention will become clearer from the following detailed description of the invention, taken with the accompanying drawings and claims, or may be learned by the practice of the invention.

These drawings are provided to illustrate various aspects of the invention and are not intended to be limiting of the scope in terms of dimensions, materials, configurations, arrangements or proportions unless otherwise limited by the claims.

While these exemplary embodiments are described in sufficient detail to enable those skilled in the art to practice the invention, it should be understood that other embodiments may be realized and that various changes to the invention may be made without departing from the scope of the present invention. Thus, the following more detailed description of the embodiments of the present invention is not intended to limit the scope of the invention, as claimed, but is presented for purposes of illustration only and not limitation to describe the features and characteristics of the present invention, to set forth the best mode of operation of the invention, and to sufficiently enable one skilled in the art to practice the invention. Accordingly, the scope of the present invention is to be defined solely by the appended claims.

In describing and claiming the present invention, the following terminology will be used.

Thus, for example, reference to "a nanoribbon" includes reference to one or more of such materials and reference to "contacting" refers to one or more such steps.

As used herein with respect to an identified property or circumstance, "substantially" refers to a degree of deviation that is sufficiently small so as to not measurably detract from the identified property or circumstance. The exact degree of deviation allowable may in some cases depend on the specific context.

As used herein, the term "at least one of" is intended to be synonymous with "one or more of. " For example, "at least one of A, B and C" explicitly includes only A, only B, only C, and combinations of each (e.g. A+B, B+C, A+C, and A+B+C).

Concentrations, amounts, and other numerical data may be presented herein in a range format. It is to be understood that such range format is used merely for convenience and brevity and should be interpreted flexibly to include not only the numerical values explicitly recited as the limits of the range, but also to include all the individual numerical values or sub-ranges encompassed within that range as if each numerical value and sub-range is explicitly recited. For example, a numerical range of about <NUM> to about <NUM> should be interpreted to include not only the explicitly recited limits of <NUM> to about <NUM>, but also to include individual numerals such as <NUM>, <NUM>, <NUM>, and sub-ranges such as <NUM> to <NUM>, <NUM> to <NUM>, etc. The same principle applies to ranges reciting only one numerical value, such as "less than about <NUM>," which should be interpreted to include all of the above-recited values and ranges. Further, such an interpretation should apply regardless of the breadth of the range or the characteristic being described.

Any steps recited in any method or process claims may be executed in any order and are not limited to the order presented in the claims. Means-plus-function or step-plus-function limitations will only be employed where for a specific claim limitation all of the following conditions are present in that limitation: a) "means for" or "step for" is expressly recited; and b) a corresponding function is expressly recited. The structure, material or acts that support the means-plus function are expressly recited in the description herein. Accordingly, the scope of the invention should be determined solely by the appended claims and their legal equivalents, rather than by the descriptions and examples given herein.

An initial overview of technology embodiments is provided below and specific technology embodiments are then described in further detail. This initial summary is intended to aid readers in understanding the technology more quickly, but is not intended to identify key or essential features of the technology, nor is it intended to limit the scope of the claimed subject matter.

Highly conductive ultrathin nanoribbons can be fabricated from an amphiphilic electron donor-acceptor supramolecule perylene tetracarboxylic diimide (PTCDI) as the backbone scaffold to enable one-dimensional intermolecular assembly via strong π-stacking. The high conductivity results from a strong donor group-substituted PTCDI, of which the strong electron donor can form a charge transfer complex with the PTCDI moiety (acting as the acceptor) of an adjacent stacked molecule, generating an anionic radical of PTCDI. Upon self-assembling into 1D nanostructures, the electron generated is delocalized along the long axis of PTCDIs through the columnar π-stacking. The resultant PTCDI radicals function as n-type dopants located in the lattice of PTCDI crystals. The self-doped one-dimensional PTCDI nanomaterial has high conductivity, combined with n-type semiconductor character, and thus makes effective chemiresistive sensors for the detection of target vapors such as oxidant gases or vapors.

A chemiresistive vapor sensor compound for detecting vapor includes a perylene-tetracarboxylic diimide (PTCDI) core according to structure (I): where R is
<CHM>
a morphology control group or -A'-D', A and A' can be independently a linking group, D and D' are independent strong electron donors which transfer electrons to the PTCDI core of an adjacent molecule of the compound sufficient to form an anionic PTCDI radical of the PTCDI core, and R1 to R8 are independent side groups or hydrogen.

The chemiresistive vapor sensor compound can be used with a chemiresistive vapor sensor for detection of a target compound. The sensor can include an assembly of nanofibers formed of the chemiresistive sensor compound and a pair of electrodes operatively oriented about the assembly of nanofibers to allow electrical current to pass from a first electrode in the pair of electrodes through the assembly of nanofibers and to a second electrode in the pair of electrodes.

Further, the chemiresistive vapor sensor can be used in a method of detecting target compounds. The method can include exposing the assembly of nanofibers to a suspected target compound source, measuring an electrical response of the assembly of nanofibers, and displaying a detection metric based on the electrical response.

It is noted that when discussing the chemiresistive vapor sensor compound, the chemiresistive vapor sensor, and the method of detecting target compounds, each of these respective discussions can be considered applicable to each of these examples, whether or not they are explicitly discussed in the context of that example. Thus, for example, in discussing details about the chemiresistive vapor sensor compound per se, such discussion also refers to the chemiresistive vapor sensor and the method of detecting target compounds, and vice versa.

With this overview in mind, the PTCDI core of the chemiresistive vapor sensor compound can include a variety of functional groups attached thereto. For example, side groups R1-R8 of the PTCDI core can typically be solubility enhancing groups, or other suitable groups that do not significantly affect the electrical properties of the PTCDI core. In some specific examples, R1-R8 can be independently selected from hydrogen, C<NUM>-C<NUM> alkyl groups that do not significantly affect the electrical properties of the PTCDI core, or a combination thereof.

Further, in some cases R can be a morphology control group. Where this is the case, the morphology control group can typically be any group that does not impair the selectivity and function of the electron donor group. In some examples, the morphology control group can be a straight chain alkyl group (e.g. a C<NUM> to C<NUM> straight chain alkyl group). In one aspect, the morphology control group can be octyl, nonyl, decyl, undecyl, dodecyl, tridecyl, tetradecyl, pentadecyl, or hexadecyl.

In some other examples, the morphology control group can be a branched C<NUM>-C<NUM> alkyl group. In some examples, the morphology control group R group can have a structure according to structure (II):
<CHM>
where n1 and n2 are <NUM>-<NUM>. In some specific examples, the morphology control group can be hexaheptyl, pentylhexyl, butylpentyl, or butyloctyl. Depending on the morphology control group employed, the chemirestistive sensor compound can have a morphology of a nanobelt, nanotubes, nanofibers, nanoribbon, or the like.

In some other examples, R is not a morphology control group. For example, in some cases, R can be -A'-D' to give the PTCDI core a structure according to structure (III):
<CHM>
where A' is a linking group and D' is (III), a strong electron donor group. It is noted that where R is -A'-D', A' can be the same linking group as A or a linking group that is different than A. Similarly, D' can be the same electron donor group as D or an electron donor group that is different from D.

Further, A and A' can generally be a carbon-based linking group. In one embodiment, A and A' can include at least one of C<NUM>-C<NUM> alkylene groups, C<NUM>-C<NUM> cycloalkylene groups, or phenylene groups. Although lengths can vary, A and A' can often be <NUM> to <NUM> or alternatively <NUM> to <NUM> carbons in length measured by carbon-carbon single bond length. In some cases, the carbon-based linking group of A and A' can include oxygen, nitrogen, and/or sulfur substitutions. For example, ether, ester or amide linkages can serve as linker groups. Non-limiting examples of substituted linking groups can include
<CHM>
<CHM>
or the like, where n can be an integer from <NUM> to <NUM> and m can be an integer from <NUM> to <NUM>.

The electron donor group, D and D', according to the invention creates a ΔG < <NUM> for formation of the anionic PTCDI radical. A variety of strong electron donor groups can be suitable. Suitable strong electron donor groups can be determined based on a variety of factors, such as oxidation and ionizing potential, nucleophilicity, steric hindrance and/or molecular geometry. Each of these factors can be used to identify strong reducing agents that are able to reduce the PTCDI core to an anionic radical. For example, D and D' can include at least one of:
<CHM>
or the like, where n can typically range from <NUM> to <NUM>, from <NUM> to <NUM>, or from <NUM> to <NUM>. In yet other examples D and D' can include at least one of:
<CHM>
<CHM>
or the like, where n can range from <NUM> to <NUM>, and in some cases <NUM> to <NUM>. In some examples, the value for n can affect the length of the linker between the donor and PTCDI core. In such cases, n can generally range from <NUM> to <NUM>, in some cases from <NUM> to <NUM>, and in one example n can be <NUM>. In yet other examples, D and D' can include at least one of:
<CHM>
<CHM>
or the like, where R'-R<NUM>' can be substituted groups that do not eliminate the electron donating ability of D and D'. For example, R'-R<NUM>' can be independently selected from the group consisting of hydrogen, C<NUM>-C<NUM> alkyl groups, C<NUM>-C<NUM> alkyl ether groups, C<NUM>-C<NUM> phenyl groups, and combinations thereof. In still other examples, D and D' can include at least one of:
<CHM>
<CHM>
<CHM>
or the like, where R''-R<NUM>'' can be substituted groups that do not eliminate the electron donating ability of D and D'. For example, R''-R<NUM>'' can be selected from the group consisting of hydrogen, C<NUM>-C<NUM> alkyl groups, C<NUM>-C<NUM> alkyl ether groups, C<NUM>-C<NUM> phenyl groups, and combinations thereof. In one specific embodiment, at least one of -A-D and -A'-D' can be
<CHM>
or the like, forming a <NUM>-methylpiperdine-substituted perylene tetracarboxylic diimide (MP-PTCDI).

Thus, a variety of R groups, linking groups, and electron donor groups can be used to prepare the chemiresistive vapor sensor compounds. For example, non-limiting examples of chemiresistive vapor sensor compounds can include:
<CHM>
<CHM>
<CHM>
<CHM>
<CHM>
<CHM>
<CHM>
<CHM>
<CHM>
<CHM>
<CHM>
<CHM>
<CHM>
<CHM>
<CHM>
<CHM>
the like, or combinations thereof.

The individual chemiresistive vapor sensor compounds can form a variety of nanostructures depending on the different functional groups employed. In some examples, the nanostructures can form via co-facial stacking of individual chemiresistive vapor sensor compounds. Thus, the nanostructures can benefit from π-π interactions and bonding between adjacent compounds along the nanostructure. In some specific examples, the nanostructures can be formed via a self-assembly process. In one specific embodiment, the nanostructure can be a nanoribbon self-assembled through columnar π-π stacking and the PTCDI radicals can function as n-type dopants located in the lattice of PTCDI crystals along the nanoribbon.

The electron transfer of the nanostructure generally occurs from the electron donor portion of one sensor compound to the PTCDI portion of another adjacent sensor compound. This is generally illustrated in <FIG>. As depicted in <FIG>, a nanofiber <NUM> can include a plurality of chemiresistive vapor sensor compounds including a PTCDI core <NUM>, an electron donor group <NUM>, and a morphology control group <NUM> (or -A'-D' group). The electron donor group, such as electron donor group 120A, of one chemiresistive vapor sensor compound according to the invention donates an electron to the PTCDI core 110A of an adjacent chemiresistive vapor sensor compound to generate an anionic radical of the electron-accepting PTCDI core 110A without photo-excitation. As such, these materials can provide sensing of target compounds in the absence of photo-excitation. This affect is at least partially facilitated by the offset stacking of the compounds within the nanofiber. For example, without being bound to any one theory, it is thought that the inter-plane stacking is usually offset through longitudinal (along long axis of molecule) or lateral sliding, or rotation along the stacking direction, mainly determined by the maximal free energy change of the stacking (relative to the free molecules). Depending on the side groups, different PTCDIs possess different configuration of stacking that gives the maximal free energy change of stacking. Regardless, the electron donor and linking groups can be sufficient to form an anionic PTCDI radical of the PTCDI core by choosing donor groups with strong electron donation ability and nucleophilicity. The linking group length can also affect charge transfer. Typically, linking groups with a larger number of carbons can increase electron donation between the electron donor compound and an adjacent PTCDI core. As such, the linking group can typically have at least three carbons, although fewer carbons can be effective depending on the strength of the electron donor group.

The chemiresistive vapor sensor compounds and associated nanostructures described herein can be used to prepare a chemiresistive vapor sensor. For example, a chemiresistive vapor sensor for detection of a target compound can include an assembly of nanofibers formed of the chemiresistive sensor compound and a pair of electrodes operatively oriented about the assembly of nanofibers to allow electrical current to pass from a first electrode in the pair of electrodes through the assembly of nanofibers and to a second electrode in the pair of electrodes. This is generally illustrated in <FIG> with a pair of interdigitated electrodes. As depicted in <FIG>, a chemiresistive vapor sensor <NUM> can include an assembly of nanofibers <NUM> formed of the chemiresistive vapor sensor compounds as described in more detail above. The assembly of nanofibers <NUM> can be deposited on a pair of electrodes including a first electrode 150A and a second electrode 150B, which can be formed on a substrate <NUM>. In some examples, the pair of electrodes 150A and 150B can be interdigitated, although other electrode configurations can be used. The donor group of one sensor compound can donate electrons to an adjacent sensor compound along a nanofiber <NUM> to generate an electrical current. As such, the electrical current can pass from the first electrode 150A to the second electrode 150B via the nanofiber assembly. Thus, in some examples, when the sensor <NUM> is exposed to a target analyte, the target analyte can interact with the nanofiber assembly <NUM> via interfacial charge transfer, which can result in a detectable change in current across the electrode pair.

The target analyte or vapor can typically be an oxidizing vapor. Although the vapor can often comprise an oxidant, generally, the target vapor can include various explosives, toxic industrial compounds, chemical warfare compounds, and the like. Specific examples of the target oxidizing vapor can include, but is certainly not limited to, at least one of peroxides (e.g. triacetone triperoxide, hydrogen peroxide, etc), nitrogen oxides (e.g. nitromethane, dinitrotoluene, trinitrotoluene, ammonium nitrate fuel oil, ammonium nitrate, PETN, RDX, etc), toxic industrial compounds (e.g. chlorine, hydrogen peroxide, sulfur dioxide, hydrochloric acid, triethyl phosphate, phosphine, hydrogen cyanide, arsine, formaldehyde, etc), chemical warfare agents (e.g. triethylphosphate, dimethyl methylphosphonate, <NUM>-chloroethyl ethyl sulfide, triphosgene, methyl salicylate, etc), and combinations thereof.

It is noted that the chemiresistive vapor sensor can include a variety of additional components or features. For example, the sensor can include a housing having an inlet and an outlet encompassing the pair of electrodes and an output electrically connected to the pair of electrodes to provide an indication of target compound detection. Additionally, in some examples, the sensor can have a forced air mechanism adapted to move air through the inlet and across the assembly of nanofibers. While photoexcitation is not necessary, the sensor can also include a light source for illuminating one or more detection zones (e.g. assembly of nanofibers). The addition of a light source can add cost and complexity, although such photoexcitation can sometimes further increase strength of detection signals.

The chemiresistive sensor can also be employed in a method of detecting target compounds. The method can include exposing the assembly of nanofibers to a suspected target compound source, measuring an electrical response of the assembly of nanofibers, and displaying a detection metric based on the electrical response. The detection metric can be one or more members selected from the group consisting of a change in electrical conductivity, change in electrical resistance, change in electrical current, and combinations thereof. The rate of change and/or recovery time varies with different analytes such that a correlation can be made using these changes and corresponding times for changes (e.g. a unique analyte response fingerprint). In one embodiment, the sensor can have a detection limit down to <NUM> ppb or lower, down to <NUM> ppb or lower, or down to <NUM> ppb or lower. Further, in some examples, the detector response time can be from about <NUM> seconds to about <NUM> seconds, about <NUM> to about <NUM> seconds, or about <NUM> seconds to about <NUM> seconds or <NUM> seconds. In many cases, the assembly of nanofibers can be responsive in the absence of exposure to light. However, in some cases, the assembly of nanofibers can be additionally exposed to light sufficient to produce distinguishable or augmented current changes upon exposure to a target compound. Depending on the target compound, the change in current can be either positive or negative.

PTCDI molecules were substituted with <NUM>-methylpiperidine (MP) to construct self-doped semiconductor through one-dimensional (1D) self-assembly of the molecules into nanoribbon structures. The methylpiperidine moiety on one molecule, acting as a strong electron donor (D), interacted with a PTCDI core on an adjacent molecule (acting as the electron acceptor, A), generating anionic radicals of PTCDI. The resultant radicals functioned as the n-type dopants located in the lattice of PTCDI semiconductors. A similar side-group induced self-doping can also be exploited in other conducting polymer materials, e.g., polyaniline. The nanoribbon structure, dominated by the π-π stacking between the PTCDI planes, provides an efficient pathway for long-range charge transport. As a result, the self-doped electrons migrated along the long axis of the nanoribbon structure, leading to enhanced conductivity. The nanoribbons can exhibit four orders of magnitude higher current as compared to 1D nanomaterials assembled from other PTCDI molecules under the same test conditions (See <FIG>).

With high conductivity, the n-type PTCDI nanoribbons can be used in a chemiresistive sensor for detection of electron deficient chemicals with a high signal-to-noise ratio, providing a reliable output signal and low limit of detection. The PTCDI nanoribbons demonstrate a sensitive chemiresistive response to hydrogen peroxide (H<NUM>O<NUM>) vapor, allowing application of the chemiresistive sensor in detection of improvised explosives, such as triacetone triperoxide (TATP), diacetone diperoxide (DADP), hexamethylene triperoxide diamine (HMTD), and simple liquid mixtures of concentrated hydrogen peroxide and fuels (e.g., alcohols, acetone). H<NUM>O<NUM> is commonly used as the chemical marker of these peroxide explosives. Current techniques for detecting H<NUM>O<NUM> are fluorometric, colorimetric, and electrochemical methods, but most of them are limited to detection in the liquid phase. It is still challenging to detect H<NUM>O<NUM> vapor at trace levels. Thus, the chemiresistive sensing technique described herein has the advantages in trace vapor detection and facilitates the fabrication of a portable, low-power, and simple sensor device.

The precursor compound, N-dodecyl-perylene-<NUM>,<NUM>,<NUM>,<NUM>-tetracarboxylic monoimide monoanhydride, was synthesized. Subsequently, the precursor compound (<NUM>), (<NUM>-methyl-<NUM>-piperidinyl)methanamine (Sigma-Aldrich, <NUM>), zinc acetate (Sigma-Aldrich, ACS reagent, <NUM>%, <NUM>) and imidazole (Sigma-Aldrich, ACS reagent, > <NUM>%, <NUM>) were combined and heated to <NUM> for <NUM> hours. After cooling to room temperature, the reaction mixture was dispersed in <NUM>% HCl (Fisher Chemical, <NUM> to <NUM>%, w/w) solution to solubilize the imidazole, the insoluble product was collected by filtration and washed with water and methanol. The crude product was converted to the free base by dissolving in chloroform and washing with <NUM>% NaOH (Sigma-Aldrich, ACS reagent, > <NUM>%) solution. The organic layer was washed with water and dried over anhydrous Na<NUM>SO<NUM> (Sigma-Aldrich, ACS reagent, > <NUM>%). The reaction mixture was then purified by silica gel chromatography using <NUM>% ethanol in chloroform as the eluent followed by recrystallization from chloroform/ethanol to give final product (<NUM>, <NUM>%).

Self-assembly of the MP-PTCDI molecules was performed through a solvent exchange process from a "good" solvent to a "bad" solvent, where the molecules have limited solubility in the "bad" solvent and thus self-assemble into one-dimensional nanostructures via molecular stacking. A solution injection method was used to conduct the self-assembly in ethanol. Typically, <NUM> of MP-PTCDI solution (<NUM> mmol/L) in chloroform was injected rapidly into a larger volume of ethanol (<NUM>) and placed in the dark for <NUM> hours. The nanoribbons were then transferred to substrates for further characterization and electrical measurements. Synthesis of MA-PTCDI and MO-PTCDI, and self-assembly into nanoribbons were performed according to similar methods.

PTCDI-based molecules have been extensively explored for 1D self-assembly and optoelectronic applications in recent years. PTCDI molecules have nodes in the π-orbitals, which allows side-chain substitutions to play an important role in intermolecular interactions, resulting in different electronic properties through charge transfer.

For example, a PTCDI molecule substituted with a <NUM>-methylpiperidine moiety was designed and synthesized as described above and as illustrated in <FIG>. The MP-PTCDI nanoribbons were fabricated using the previously reported solution phase self-assembly method. The morphology of MP-PTCDI nanoribbons was characterized by SEM and AFM (<FIG>). The nanoribbons are several micrometers in length and <NUM>-<NUM> in width. The thickness of the nanoribbons is estimated to be just about <NUM> (<FIG>, AFM image and line-scan profile). Such shape-defined 1D nanoribbon structures are conducive to the construction of electronic devices. For comparative study, two other PTCDI molecules, MA-PTCDI and MO-PTCDI (both substituted with the same dodecyl alkyl chain, but with different groups on the other end, were also synthesized and assembled into nanoribbon structures. Owing to the similar molecular structure, the two reference PTCDIs formed about the same nanoribbon morphology as the MP-PTCDI. The dimethylaniline moiety of MA-PTCDI acts as a strong electron donor (under photoexcitation), whereas the methoxyphenyl is a less effective donor to PTCDI.

For materials characterization, UV-Vis absorption spectra were collected with an Agilent Cary <NUM>. Fluorescence spectra were acquired on an Agilent Eclipse spectrophotometer. The bright field and fluorescence optical images were obtained with a Leica DMI4000B inverted microscope equipped with an Acton SP-<NUM> Imaging Spectrograph system and Princeton Instrument Acton PIXIS: 400B Digital CCD Camera System for high resolution imaging. AFM measurements were carried out on a Veeco MultiMode V scanning probe microscope in tapping mode. SEM measurement was performed with an FEI Nova Nano <NUM> (FEI Corporation) with a helix detector in low vacuum (<NUM> Torr water pressure). To make samples for either AFM or SEM measurements, the MP-PTCDI nanoribbons were directly transferred from ethanol and deposited onto a silicon substrate coated with a polished <NUM> thick SiO<NUM> layer, and then dried in vacuum oven at room temperature in the dark.

For current measurement, interdigitated electrodes (IDE) were used for all current measurements. The IDE has a channel width of <NUM> and a gap length of <NUM>, and was fabricated by a standard photolithography procedure on a silicon wafer with a <NUM> thermal oxide layer (Silicon Quest International). The electrodes were made by sputtering with <NUM> titanium adhesion layer and <NUM> gold layer. MP-PTCDI nanoribbons were deposited onto IDE by drop-casting, followed by drying in vacuum oven at room temperature in the dark. The electrical conductivity was measured under ambient conditions using a two-probe method on a Signatone S-<NUM> Probe Station combined with an Agilent 4156C Precision Semiconductor Analyzer. To compare the conductivity of different PTCDI nanomaterials, <NUM> nmol PTCDI nanomaterials were deposited onto the IDE by drop-casting and the conductivity was tested under the same conditions. To compare the current enhancement ratio after surface coating with amines, <NUM> nmol of CH-PTCDI nanobelts were chosen as the standard, zero-doping material, and deposited onto the IDE. <NUM>µL of methanol solution containing different concentrations of the amines (<NUM>-methylpiperidine, TCI America, > <NUM>%; hexylamine, Acros Organics, <NUM>%; triethylamine, Sigma-Aldrich, <NUM>%; aniline, Acros Organics, <NUM>%, extra pure) were drop cast onto the surface of the CH-PTCDI nanobelts, providing the varying molar amount of amine coated on the surface. To avoid the oxidation of amines during processing, the fresh amine solution was made for each measurement. <NUM>H NMR measurements were conducted for all the amines during the project period to assure that the high purity of amines remained throughout the experiments.

MP-PTCDI nanoribbons possess a high electrical conductivity. For example, the current of the MP-PTCDI nanoribbons is four orders of magnitude higher than MA- and MO-PTCDI nanoribbon materials under the same test conditions. Considering the similar nanoribbon structures formed from other PTCDIs and the fact that the π-electronic property of PTCDI backbone remains unchanged with different side-substitutions, it is suspected that the high conductivity observed for MP-PTCDI nanoribbons is largely caused by the methylpiperidine moiety.

To gain insight into the high conductivity of MP-PTCDI nanoribbons, a case study model was constructed by coating <NUM>-methylpiperidine molecules on N,N'-di(cyclohexyl)-perylene-<NUM>,<NUM>,<NUM>,<NUM>-tetracarboxylic diimide (CH-PTCDI) nanobelts to investigate the influence of this specific substitution group on the conductivity of one-dimensional PTCDI nanomaterials. The CH-PTCDI was selected because the cyclohexyl side-chain groups are neutral and inactive in charge transfer interactions, and the shape defined nanobelts can be easily fabricated from this molecule with high reproducibility. In this study, <NUM> nmol of CH-PTCDI nanobelts were deposited onto interdigitated electrodes (IDEs) patterned on a silicon wafer, and a controlled amount of <NUM>-methylpiperidine was drop-cast onto the nanobelts. Negligible currents were measured for either pristine CH-PTCDI nanobelts (<NUM> nA at a bias of <NUM> V, <FIG>) or pure <NUM>-methylpiperidine film drop-cast from <NUM>µmol amount (<NUM> nA at a bias of <NUM> V, <FIG>). In contrast, a much increased current was observed for the CH-PTCDI nanobelts coated with only <NUM>µmol of <NUM>-methylpiperidine (<NUM> nA at a bias of <NUM> V, <FIG>). The current increased further when more <NUM>-methylpiperidine was deposited. The current increased to <NUM>µA, an enhancement ratio of <NUM>×<NUM><NUM> compared to the pristine nanobelt, when <NUM>µmol of <NUM>-methylpiperidine was deposited (<FIG>). An enhancement ratio of <NUM>×<NUM><NUM> was observed when <NUM>µmol of <NUM>-methylpiperidine was deposited, indicating that the current increase started to reach saturation when more than <NUM>µmol of <NUM>-methylpiperidine was cast (<FIG>). This significant increase in conductivity is likely due to the chemical doping, which occurs through the donor-acceptor (charge transfer) interaction between <NUM>-methylpiperidine and PTCDI.

The charge transfer interaction between <NUM>-methylpiperidine and CH-PTCDI was also confirmed by the fluorescence quenching measurements of PTCDI. MP-PTCDI solution in chloroform exhibits strong fluorescence emission, comparable to CH-PTCDI molecules in chloroform (<FIG>), indicating that there is no intramolecular electron transfer in the MP-PTCDI molecule. However, no fluorescence emission was observed for the nanoribbons fabricated from the MP-PTCDI molecules (<FIG>). By contrast, the CH-PTCDI nanobelts still have considerable fluorescence emission (<FIG>). The fluorescence quenching within MP-PTCDI nanoribbons is likely caused by the intermolecular electron transfer between <NUM>-methylpiperidine on one molecule and the PTCDI part on the other molecule. To prove this intermolecular charge transfer, fluorescence spectra of the reference PTCDI, CH-PTCDI, were measured in chloroform solutions with and without addition of <NUM>-methylpiperidine (<FIG>). Significant fluorescence quenching (indicative of charge transfer interaction) was observed with increasing concentrations of <NUM>-methylpiperidine, which follows the linear Stern-Volmer relationship (<FIG>). The linear fitting gives the binding constant of <NUM>-<NUM> between <NUM>-methylpiperidine and the PTCDI. The binding constant obtained is about one order of magnitude higher than those measured for the aromatic hydrocarbon donor-acceptor complexes. The enhanced binding is largely due to the stronger electron donating capability of organic amines, compared to the aromatic hydrocarbons.

To determine whether other amines can interact with PTCDI as strong as <NUM>-methylpiperidine, aniline, hexylamine, and triethylamine were selected for comparative testing. Following the same experimental procedure of surface doping as described above, different amounts of amines were drop-cast onto the surface of the CH-PTCDI nanobelts. As shown in <FIG>, when <NUM>µmol of amine was applied, no obvious current enhancement was obtained by coating with aniline, hexylamine, and triethylamine, whereas a <NUM>-fold increase in current was observed by coating with <NUM>-methylpiperidine under the same conditions. However, a significant increase in current was observed when the amount of hexylamine and triethylamine increased to <NUM>µmol, for which the enhancement ratios were about <NUM> and <NUM>, respectively (though still four orders of magnitude lower than <NUM>-methylpiperidine), whereas aniline still produced negligible current even with as much as <NUM>µmol coated (<FIG>).

Regarding the fact that the CH-PTCDI nanobelts coated by the three amines (<NUM>-methylpiperidine, hexylamine, and triethylamine) produced enhanced conductivity relative to the pristine CH-PTCDI nanobelts while those treated with aniline did not, the interaction between CH-PTCDI molecules and amines using UV-Vis absorption spectroscopy was studied. The PTCDI anionic radical upon adding the three amines into the oxygen free solution of CH-PTCDI in dimethyl sulfoxide (DMSO) was detected. The existence of three new absorption peaks at <NUM>, <NUM>, and <NUM> in the presence of <NUM>-methylpiperidine indicated the formation of PTCDI anionic radicals (<FIG>). In the absence of oxygen, the radicals are very stable. Upon exposure to air, the three characteristic peaks diminish with time, which further confirms the formation of oxygen-sensitive PTCDI anionic radicals. Moreover, analysis based on the redox potentials suggests that the electron transfer from <NUM>-methylpiperidine to PTCDI is a thermodynamic favorable (spontaneous) process. In addition to <NUM>-methylpiperidine, PTCDI anionic radicals were also generated by addition of hexylamine and triethylamine (<FIG>). However, no such anionic radicals were generated upon addition of aniline to the same deoxygenated solution of CH-PTCDI (<FIG>). This comparative observation is consistent with the above discussed results of conductivity enhancement upon casting of different amines, indicating that the effective charge transfer interaction (and thus generating PTCDI anionic radical) is the primary cause of the conductivity enhancement. The electrons thus generated can delocalize (migrate) along the π-π stacking of PTCDIs, acting as the major charge carrier for the n-type material.

The electron donating strength of amines can be evaluated by the oxidation potentials. The selection rule of donor relies on a thermodynamics analysis, i.e., the Gibbs free energy change (ΔG) of the electron transfer process forming the anionic radical must be negative, ΔG < <NUM>. The Gibbs free energy change (ΔG) of the electron transfer can be calculated from the redox potentials of species under certain concentrations; ΔG thus obtained will indicate whether the electron transfer is a thermodynamically favorable (spontaneous) process, for which ΔG < <NUM>. The oxidation potential of <NUM>-methylpiperidine (MP) is <NUM> V vs SCE, and the reduction potential of PTCDI is -<NUM> V vs SCE (Note: the electronic property (redox potential) of PTCDI does not change significantly with the different side groups since the two imide positions are nodes in the π-conjugation). For example, the starting concentration of PTCDI and MP used in our solution phase UV-vis spectral measurement were <NUM>µmol/L and <NUM> mol/L, respectively. When the redox reaction reached its equilibrium as shown in <FIG>, the concentration of PTCDI anionic radical can be estimated to be about <NUM>×<NUM>-<NUM> mol/L, according to Beer-Lambert law (given the molar absorption coefficient of PTCDI anionic radical is ε = <NUM>/mol cm-<NUM> at <NUM>, peak absorbance = <NUM> at that wavelength, optical length = <NUM>). The concentration of the counterpart (cationic ion) MP+ should be the same as that of the PTCDI anionic radical, <NUM>×<NUM>-<NUM> mol/L. So, from the Nernst equation XX, and T = <NUM>, the electrical potential (ΔE) of the redox (electron transfer) reaction between MP and PTCDI can be calculated (below) to be <NUM> V, which gives a negative ΔG, meaning a spontaneous process for the electron transfer in the solution phase. <MAT> <MAT> where R is the universal gas constant, z is the number of electrons transferred per ion pair, F is the Faraday constant, [D] is the concentration of donor group, [D+] is the concentration of the donor cationic ion, [PTCDI] is the concentration of PTCDI, and ΔE<NUM> is the electric potential difference of the PTCDI and donor groups. Considering the tight intermolecular packing within MP-PTCDI nanoribbons (pi-pi stacking distance between PTCDI planes is about <NUM>Å), the local concentration of MP and PTCDI in the solid would be much higher than that in the solution phase. Therefore, the Gibbs free energy change (ΔG) of the electron transfer should be increased (becoming more negative), meaning the generation of PTCDI anionic radicals should be more favored in the nanoribbons. So for the donors without experimental data, it is expected that whenever the electrical potential (ΔE) of the electron transfer reaction between donor and PTCDI in the molecular assembly (at certain concentration) is positive, meaning the ΔG is negative, the donor group would satisfy the criteria for self-doping to donate an electron into PTCDI.

Among all the amines used, aniline would be the strongest electron donor from their oxidation potentials (<NUM>-methylpiperidine, Eoox = <NUM> V vs SCE; hexylamine, Eoox = <NUM> V vs SCE; triethylamine, Eoox = <NUM> V vs SCE; aniline, Eoox = <NUM> V vs SCE). However, as evidenced by these results, no charge transfer was observed between aniline and PTCDI even in the presence of a large excess of aniline (i.e., no significant current enhancement was observed with <NUM>µmol aniline coated on the CH-PTCDI nanobelts, and no absorption peaks of PTCDI anionic radical were detected in the deoxygenated DMSO solution of CH-PTCDI (<NUM>µmol/L) in the presence of excessive aniline (<NUM> mol/L)). The lack of charge transfer between aniline and PTCDI might be due to the weak nucleophilicity of aniline, which prevents the strong donor-acceptor interaction.

On the basis of the aforementioned results and discussion, an n-type doping mechanism, which is a result of the generation of PTCDI anionic radicals, explains the high conductivity of the MP-PTCDI nanoribbons. As demonstrated in <FIG>, upon self-assembly into nanoribbons, the side-groups of <NUM>-methylpiperidine are in close proximity of the PTCDI backbones, enabling charge transfer interaction to form PTCDI anionic radicals (the reduction of PTCDI by <NUM>-methylpiperidine can be more thermodynamically favored in solid state compared to solution phase due to a much higher local concentration in solid). Owing to the efficient intramolecular π-electron delocalization within the PTCDI plane, the electron (anionic radical) generated can be well stabilized (against charge recombination) as observed in the UV-Vis absorption spectral measurement (<FIG>). When this occurs inside the nanoribbons, the electron can effectively survive scavenging by oxygen, making the high conductivity gained sustainable even in the ambient environment as indeed observed in this study (<FIG>). With an applied bias, the self-doped electrons rapidly migrate along the long axis of the nanoribbon facilitated by the intermolecular π-π electron delocalization, leading to the high conductivity. The resultant PTCDI anionic radical is an n-type dopant in which the substitutional dopant is a zwitterionic molecule, a PTCDI anionic radical linked to an amine centered cation (a reduced analogue of the PTCDI host molecule). An n-doped PTCDI film was fabricated by spin-coating mixed PTCDI dopant and host materials solution, resulting in ten orders of magnitude of increase in conductivity with just <NUM>% doping.

The morphology of the CH-PTCDI nanobelts before and after surface coating remained unchanged as characterized by SEM (<FIG>). Interestingly, the built up surface charging on the pristine CH-PTCDI nanobelts was eliminated by surface coating with <NUM>-methylpiperidine. The SEM image of pristine CH-PTCDI nanobelts shows bright imaging contrast on the surface of the nanobelts, which is a characteristic of the surface charge built up on the nonconductive sample after E-beam exposure during SEM measurement (<FIG>). However, such surface charging was suppressed after surface coating of the <NUM>-methylpiperidine (<FIG>). This phenomenon is common in SEM measurements performed on nonconductive samples, for which a thin layer of metal, such as Au, Pt, or Pd, is typically deposited on the sample surface to facilitate charge transmission and prevent charge building up. Since the pure <NUM>-methylpiperidine film is not overly conductive (<FIG>), the observed conductivity improvement should be due to the charge transfer interaction between <NUM>-methylpiperidine and CH-PTCDI nanobelts, consistent with the current enhancement discussed above.

All chemical vapor sensing tests were conducted under ambient conditions in the dark. <FIG> depicts a sensing and chemical vapor delivery system. A chemical vapor was pulled into a <NUM> syringe and delivered by syringe pump <NUM> (NE-<NUM> New Era Pump System, Inc. ) at a rate of <NUM>/min into the carrier gas. The carrier gas was dry air delivered by a carrier gas container <NUM> and mass flow controller <NUM> at a flow rate of <NUM>/min. The final concentration of chemical vapor in the testing chamber <NUM> was calculated from the syringe volume and the concentration of original chemical vapor. The original H<NUM>O<NUM> vapor was generated from <NUM> wt. % H<NUM>O<NUM> solution (Sigma-Aldrich, <NUM> wt. % in H<NUM>O). The IDE chip <NUM> (deposited with PTCDI nanomaterials) was placed on a ceramic chip carrier connected by wire bonding. The ceramic chip carrier was fixed on a breadboard, enclosed in a small Teflon chamber (<NUM> in length, <NUM> in width, and <NUM> in height), and connected to an Agilent 4156C Semiconductor Analyzer <NUM>. A bias of <NUM> V was applied across the electrodes and the current through the sensor was monitored. For H<NUM>O<NUM> testing, the chip was exposed to H<NUM>O<NUM> vapor for <NUM> with a recovery time of <NUM>. For testing toward the common liquid samples, the exposure time was <NUM> with a recovery time of <NUM>. The sensing response time of MP-PTCDI nanoribbons was obtained by fitting the time-course current change profile to an exponential function, which gives the response time as <NUM>/e of the time constant obtained.

With the increased electrical conductivity by self-doping, MP-PTCDI nanoribbons can be a building material for chemiresistive sensors, for which the high conductivity improves the signal-to-noise ratio and simplifies the system design. A chemiresistive sensor based on the nanoribbons benefits from the large surface area and continuous porosity formed by the interlaced nanoribbons deposited on the substrate (<FIG>). Combination of these two features enhances the adsorption and diffusion (accumulation) of gas analytes, thus increasing the sensing sensitivity. The n-type character of the PTCDI material allows for chemiresistive sensing of electron deficient analytes, which can be bound to the surface, causing charge depletion of the material. H<NUM>O<NUM> vapor was chosen as the target analyte because it is a critical signature for the peroxide explosives including both the synthetic ones (e.g., TATP, DADP and HMTD) and the simple liquid mixtures of H<NUM>O<NUM> and the fuels.

<FIG> and <FIG> show the real-time electrical current profile of an MP-PTCDI nanoribbon chemiresistor sensor in response to H<NUM>O<NUM> vapor. Upon exposure to H<NUM>O<NUM> vapor (<NUM> ppm), there is an instantaneous decrease in current of about <NUM>%. A very short response time of <NUM> seconds is attributed to the large surface area of the nanoribbons and expedient diffusion of H<NUM>O<NUM> vapor. The response is concentration dependent. <FIG> shows a plot of relative sensor response as a function of the concentration of H<NUM>O<NUM> vapor, which can be fit well into the Langmuir absorption model. The lowest concentration of H<NUM>O<NUM> vapor that was tested in this study was <NUM> ppm, which represents the lowest level that can be provided by the present experimental setup. Nonetheless, the limit of detection of H<NUM>O<NUM> vapor can be projected to be <NUM> ppb following the Langmuir adsorption model. The irreversible sensor response towards H<NUM>O<NUM> is attributed to the strong surface binding of H<NUM>O<NUM> and the permanent oxidation of <NUM>-methylpiperidine groups by H<NUM>O<NUM> (E°red =<NUM> V, vs SCE).

In addition to the high sensitivity, the MP-PTCDI nanoribbons also demonstrated high selectivity towards H<NUM>O<NUM> vapor against water and some common organic liquids, facilitating the development into practical sensing applications. Such general selectivity was investigated by measuring the sensor response toward the vapor of various common liquids, including water, acetone, ethyl acetate, dichloromethane, methanol, ethanol, toluene, and hexane. In contrast to the irreversible decrease response caused by H<NUM>O<NUM> vapor, exposure to these liquids vapor resulted in reversible increase in current for the MP-PTCDI nanoribbons (<FIG>). The vapors exhibited fluorescence quenching (without photoexcitation) in increasing degree from lower to higher, respectively, with hexane, toluene, ethanol, water, methanol, dichloromethane, ethyl acetate and acetone as shown in <FIG>. The increased conductivity observed is likely a result of dipole interaction between MP-PTCDI nanoribbon and the liquid molecule. The sensing response and the dipole moment of the liquids appear to be tightly correlated. For liquids with smaller dipole moments, the response is lower. For example, the vapor concentration of hexane used (<NUM> ppm) is much higher than many other analytes, such as ethyl acetate (<NUM> ppm), water (<NUM> ppm), ethanol (<NUM> ppm), toluene (<NUM> ppm), but the relative response is only <NUM>%, the lowest among all chemicals studied here, because the dipole moment of hexane is less than <NUM> Debye, much lower than others.

In conclusion, the nanoribbons assembled from the <NUM>-methylpiperidine substituted-PTCDI molecules possess extraordinarily high conductivity relative to other PTCDI-based nanostructures. The <NUM>-methylpiperidine group plays a key role in the conductivity enhancement, as evidenced by systematic experiments and analysis of the interaction between a model PTCDI nanobelt and <NUM>-methylpiperidine. Upon self-assembly into one-dimensional nanoribbons, the <NUM>-methylpiperidine groups interact with the PTCDI core in stacking proximity to produce the PTCDI anionic radical, which acts as the n-type dopant in the PTCDI lattice. The doping process increases the charge carrier density within the PTCDI nanoribbons, and the one-dimensional π-π stacking of PTCDIs is efficient for long range charge migration, thereby resulting in high conductivity. The high conductivity obtained supports application in chemiresistive sensors. The PTCDI nanoribbons demonstrated highly sensitive response to H<NUM>O<NUM> vapor through oxidation, rather than dipole moment interaction as in the case of common liquid vapor, thereby producing general selectivity toward H<NUM>O<NUM> vapor. Owing to the high conductivity of MP-PTCDI nanoribbons, as well as the porous mesh-like morphology of the nanoribbon film, the lowest detected concentration of H<NUM>O<NUM> vapor in this study was down to <NUM> ppm, and the limit of detection is projected to be as low as <NUM> ppb.

Claim 1:
A chemiresistive vapor sensor compound for detecting target vapors, comprising a perylene-tetracarboxylic diimide (PTCDI) core according to structure I:
<CHM>
where R is a morphology control group or -A'-D', A and A' are independent linking groups;
wherein D and D' are independent strong electron donors which transfer an electron to the PTCDI core of an adjacent molecule of the compound sufficient to form an anionic PTCDI radical of the PTCDI core of the adjacent molecule, with both D and D' creating a Gibbs free energy change ΔG < <NUM> for formation of the anionic PTCDI radical that facilitates the formation of the anionic PTCDI radical without photo-excitation; and
R1 to R8 are independent side groups or hydrogen.