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Dr Hong Zhang, associate professor at the Van’t Hoff Institute for Molecular Sciences, was invited to contribute a tutorial review to the highly regarded magazine Chemical Society Reviews. It will be published in a coming themed issue on "Photon Upconversion Nanomaterials". These materials provide interesting opportunities for improving biomedical imaging and therapy as well as solar energy utilization. Zhang describes the state-of-the-art science of energy upconversion mechanisms in spatially confined systems and discusses the challenges lying ahead.

Excitation energy migration in a lanthanide ions doped upconversion nanoparticle.
Excitation energy migration in an upconversion nanoparticle. Image: Hong Zhang

Upconversion is a process in which one photon is emitted upon absorption of several photons of lower energy. It thus 'jacks' the light from lower to higher frequencies. Typically upconversion materials are doped with lanthanide ions. These are able to shift the near infrared (NIR) light of an economic continuous wave milliwatt laser towards higher, visible frequencies and even into the ultraviolet (UV) spectral region.

Biomedical application

"In recent years new nanometer-sized upconversion materials have been boosting scientific interest", says Hong Zhang. "Since they provide excitation in the so-called “optical window” of human tissue an important research focus lies in biomedical application. Significant improvement is expected in optical imaging, labelling and therapy."

Already a myriad of in vitro and in vivo tests have been published according to Hong Zhang. Upconversion nanomaterials have been successfully used for the detection of nucleic acids on microarrays, for diagnosis and therapy of various infectious diseases, for optical chemical sensors and for bioimaging.

"Especially in homogeneous assays in complex biological samples such as blood, the use of upconversion nanomaterials as an alternative to common molecular fluorophores leads to much higher detection sensitivities", the HIMS researcher explains. "This is because upconversion benefits from the lack of background fluorescence under near infrared (NIR) excitation. On the other hand the use of upconversion nanomaterials in bioimaging has extended the application range of conventional fluorescence microscopy to deep tissue imaging and photodynamic therapy."

Enhancing efficiency

An important issue with the current lanthanide doped upconversion materials is the low efficiency (less than 1%) at the desired low power densities (below 150 W/cm2). This is a major concern for actual biomedical application, according to Hong Zhang. For example, the depth of tissue imaging depends to a large extent on the upper limit of the excitation power density, which in turn depends on the upconversion efficiency.

Adding to this, the current upconversion efficiency is also the bottleneck when considering any possible application in solar energy utilization. Here the promise of upconversion materials lies in enhancing the efficiency of photovoltaic cells by extending the range of solar frequencies that can be converted to electricity.

According to Hong Zhang, nanostructuring the upconversion materials offers great opportunities both for improving the upconversion efficiency and for understanding the upconversion mechanism. In his review he covers the significant progress that has been made in the comprehension of upconversion dynamics in nanosystems, such as the role of the surface, the long-range migration and the external field. This has already led to several strategies for the reduction of excitation energy loss as well as the enhancement of radiative and nonradiative transition probabilities.

H. Zhang
Dr Hong Zhang

Hong Zhang

Dr Hong Zhang is both associate professor in the Molecular Photonics Group of the Van 't Hoff Institute for Molecular Sciences (UvA) and professor at the Changchun Institute of Optics, Fine Mechanics and Physics of the Chinese Academy of Sciences (CAS). He thus leads two research groups both at UvA and CAS. Hong Zhang also holds a chair at the Northeast Normal University in Changchun (China).

His research focuses on the excitation energy migration dynamics in spatially confined systems and the development of bio-functionalized upconversion nanomaterials.  At the Van 't Hoff Institute for Molecular Sciences Hong Zhang and colleagues perform their research in close collaboration with the Academic Medical Center Amsterdam (AMC), Leiden University Medical Center (LUMC) and the Chinese Academy of Sciences. Relevant research activities have been supported by the Innovation Oriented Program (IOP) of AgentschapNL and the China Exchange Programs of the  Royal Netherlands Academy of Arts and Sciences (KNAW), and by the Natural Science Foundation of China and the State Key Laboratory of Luminescence and Applications in China. Relevant papers of the group have been cited for more than 2300 times in recent years. 

Tutorial review

“Excitation Energy Migration Dynamics in Upconversion Nanomaterials”, Langping Tu, Xiaomin Liu, Fei Wu, Hong Zhang, Chem. Soc. Rev., (2014), DOI:10.1039/c4cs00168k.