Photochemistry

AIE Detectives Make Life Safer

by Ben Zhong Tang

October 31, 2016

Frank Morn’s 1982 history of the famous Pinkerton detective agency is titled The Eye That Never Sleeps. Indeed, a responsible detective must keep his or her eyes wide open to observe, analyze, and take timely action.

A chemist detective, however, cannot detect many harmful conditions with the naked eye alone. Detection systems with high sensitivities and fast responses are needed to assist the detective. Here are two cases in which researchers used fluorogens that have the property of aggregation-induced emission (AIE) to create fluorescent systems that sensitively detect microscopic mechanical damage or vapors of volatile organic compounds.

Self-reporting damage

Microscopic damage to polymers and composites is difficult to spot, yet it compromises mechanical integrity and inevitably leads to failure. Detecting small-scale damage before catastrophic failure is important to ensure the safety and reliability of critical engineering components and reduce the costs of regular maintenance.

Developing self-reporting detection systems that continuously monitor damage is appealing because no human intervention is required. Nancy R. Sottos, Jeffrey S. Moore, and co-workers at the University of Illinois at Urbana–Champaign invented a simple, versatile AIE-based system that sensitively detects mechanical damage.

The authors assembled the self-reporting system with autonomous damage indication by embedding core–shell microcapsules that contain a dilute solution of tetraphenylethylene (TPE), a simple AIE luminogen, in a polymeric material (Figure 1). When the material is damaged, the microcapsules rupture, and the encapsulated solution is released. Subsequent evaporation of the solvent causes the TPE molecules to aggregate and emit visible fluorescence when irradiated with UV light. The light emission intensifies rapidly after the mechanical damage occurs and reaches its maximum intensity in minutes.

Figure 1

This powerful sensing system relies on only one active component (TPE) and should perform well in a variety of materials. In contrast to alternative methods, this detection system is widely applicable, does not rely on external or intermolecular interactions to elicit a response, and sensitively provides outstanding contrast between the intact and damaged regions. The advances in encapsulation chemistry, the availability of diverse AIE luminogens, and the easy incorporation of microcapsules into existing material formulations make this technology readily accessible. (ACS Cent. Sci. DOI: 10.1021/acscentsci.6b00198)

Sensing VOCs

Volatile organic compounds (VOCs), small organic molecules that evaporate under ambient conditions, are numerous and ubiquitous. VOCs are the most common odors that people can detect in the atmosphere. Many of them are detrimental to human health and/or harmful to the environment. 

Figure 2

VOCs can be detected by a variety of analytical methods; the most prominent is mass spectrometry. This equipment, however, is expensive and can be operated only by skilled technical personnel. In addition, data interpretation and structure determination are often complex. Seeking a more practical sensor method, Yang Liu and colleagues at Shandong University (Jinan, China) developed a convenient fluorescence technique for sensing VOCs that uses AIE luminogen 1 in Figure 2.

The researchers prepared the luminogen by attaching two electron-donating (D) TPE units to an electron-accepting (A) dicyanopyrazine core. The resulting D–A–D molecular structure makes 1 an intramolecular charge-transfer molecule. The color of its emitted light varies from red (crystalline) to yellow (amorphous), depending on the aggregate’s morphology.

The authors demonstrated how the polymorphs switch from one to another by fuming solid films of 1 with vapors of several widely used solvents (see “Common VOCs”). By using its reversibly switchable, morphology-dependent fluorescence, a phenomenon known as vapochromism, the authors used 1 as a visual sensor for detecting VOCs.

When crystalline films of 1 were exposed to the vapors, their light emission color changed from red to yellow at a characteristic rate. For example, under UV irradiation, toluene, tetrahydrofuran, and chloroalkanes turned films yellow in 1 min, whereas films exposed to n-hexane, methanol, and dimethyl sulfoxide did not change color even after 10 min. Additional AIE luminogens are needed to detect these compounds.

Common VOCs

n-Hexane Methanol
Cyclohexane Dimethyl sulfoxide
Toluene Carbon tetrachloride
Ethyl acetate Dichloromethane
Tetrahydrofuran Ethylene dichloride
Acetone Chloroform
Dimethylformamide  

The sensing response is the result of the interaction between the crystalline lattice and the VOC adsorbent. Disturbing the molecular packing of 1 by the adsorbed VOC molecules converts the structure from crystalline to amorphous and causes the color change. The authors verified this structural modification by observing changes in the X-ray diffraction patterns of films of 1 after they were exposed to the VOC vapors. (Mater. Chem. Front. DOI: 10.1039/c6qm00146g)

Life becomes safer

Invisible microscale mechanical damage can lead to disasters: for example, aging bridges with accumulated undetectable small cracks can collapse, and airplanes with strained or fatigued parts can come apart in flight. And because they are invisible, VOCs are a hidden threat to health and the environment.

Readily visible warning signals given by the sensitive AIE luminogens under continuous operation allow people to spot and repair damage and eliminate VOCs well in advance of trouble, avoiding catastrophic consequences. With the aid of AIE “eyes” that never sleep, the world will become safer!