3D Printable Vapochromic Sensing Materials
Authors: Stevens David M. , Gray Bonnie L. , Leznoff Daniel B. , Furukawa Hidemitsu , Khosla Ajit 
Published: May 2020

Vapochromic Coordination Polymers (VCPs) are highly stable sensing compounds that can be used for chemical sensors but require immobilization to be effective. A novel immobilization method for VCPs has been developed that results in a new class of Vapochromic Sensing Materials (VSMs) for chemical sensors. These VSMs can further be used as base materials for 3D printing and additive manufacturing processes to create geometrically complex sensor surfaces, or for integration with other 3D printed structures. The VCP Zn[Au(CN)2]2 is used along with polylactic acid (PLA) to create the first type of this new class of VSMs. The method is simple, robust, and employs the use of the inexpensive and sustainable 3D printing polymer PLA. Early results suggest that, compared to un-immobilized VCPs, the VSM: 1) provides long term and stable immobilization of VCPs; 2) can detect target analytes (i.e. ammonia) at low concentrations (i.e. 1 ppm); and 3) is effective as a sensing material even when comprised with low VCP concentrations of 2% wt. or less.

Index Terms— 3D printing, chemical sensors, immobilization techniques, PLA polymer, vapochromic coordination polymers, vapochromic sensing materials.

Vapochromic coordination polymers (VCPs) are metal-ligand based supramolecular materials constructed primarily from coordination bonding that exhibit changes in their spectroscopic signature in response to exposure to target analytes. The difficulty in immobilizing VCPs is due to many VCPs being insoluble in most solvents. Additionally, for the VCPs to remain effective, any immobilization method that is used needs to refrain from restricting analyte access to the near total surface area of the VCP crystals to allow for maximal bonding to occur. Another requirement for the immobilization of the Zn[Au(CN)2]2 VCP is that any materials or polymers used to immobilize it must not absorb or fluoresce between the wavelengths of 400 to 550 nm; this is because it would interfere with the excitation and fluorescence response of the VCP. Our previous work on immobilizing the Zn[Au(CN)2]2 VCP has been through the micromachining of polydimethylsiloxane (PDMS) post arrays, and drop casting a VCP solvent mixture into the post array [1]. While this created sensor surfaces that could detect the target gas (i.e. ammonia) at a concentration of 5 ppm when using the Zn[Au(CN)2]2 VCP, it did not provide definitive long term immobilization and it appears to lose sensitivity over time.
Through the use of non-solvent induced phase separation (NIPS) we now develop a method to immobilize the Zn[Au(CN)2]2 VCP with PLA to create novel vapochromic sensing materials (VSMs) that can be used directly as sensor surfaces, or converted to feeder stock for additive manufacturing processes (e.g., 3D printing). By doing this, we create long term sensors surfaces that immobilize VCPs while not rendering them inert in the presence of ammonia. In order to fabricate the sensing material, we employ a modified process originally developed for creating porous polylactic acid monoliths [2]. PLA is mixed with the solvent dichloromethane (DCM) to form a homogeneous solution. The VCP is then mixed into the PLA/DCM solution until it is uniformly mixed, at which point n-Hexane is added and vigorously mixed to induce phase generation and the creation of a porous hydrogel. The hydrogel is then transferred to an evaporation dish and topped with an equal volume ratio solution of DCM and n-Hexane. Methanol is then added to act as solvent exchanger and covered loosely and allowed to evaporate over 24-48 hours. After complete evaporation and resultant solidification, the new vapochromic sensing material still requires activation. Activation is achieved through exposure to gas drawn from the headspace of a 28% Ammonia Hydroxide bottle and sealed for a period of 1-2 hours; afterwards, the VSM is left open in a fume hood for another 24-48 hours to facilitate the return to its resting state. Ammonia detection in the range of 1 ppm and below is possible with the most porous sensing materials using our experimental apparatus and algorithms [3, 4]. Figure 1 shows both a VSM sample and the VSM surface under different magnifications after the synthesis process is complete. Figure 2 shows the optical response of the VSM upon exposure to different concentrations of ammonia.
References

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Figure 1