Surface-based molecular self-assembly: Langmuir-Blodgett films of amphiphilic Ln(III) complexes
© The Author(s) 2016
Received: 4 June 2016
Accepted: 23 November 2016
Published: 28 November 2016
The unique photophysical properties of the Ln(III) series has led to significant research efforts being directed towards their application in sensors. However, for “real-life” applications, these sensors should ideally be immobilised onto surfaces without loss of function. The Langmuir-Blodgett (LB) technique offers a promising method in which to achieve such immobilisation. This mini-review focuses on synthetic strategies for film formation, the effect that film formation has on the physical properties of the Ln(III) amphiphile, and concludes with examples of Ln(III) LB films being used as sensors.
KeywordsLanthanides Langmuir Langmuir-Blodgett Surface Sensors Self-assembly Amphiphilic Luminescence Ln(III)
The construction of lanthanide-based functional nanostructures is an active area of research. Trivalent lanthanide ions have readily manipulated coordination environments and interesting photophysical properties (e.g. sharp, long-lived emission at long wavelengths) making them particularly useful in molecular recognition and sensing [1–5]. The majority of studies have been carried out in solution, however to progress towards practical, robust and commercialised sensing applications (e.g. personal sensors or medical devices) these complexes should ideally be on a surface. As such there has been significant effort directed towards functionalising Ln(III) complexes with groups for surface attachment, including the formation of amphiphilic Ln(III) systems for Langmuir-Blodgett (LB) deposition.
Synthesis of Ln(III) amphiphiles and strategies in film formation
In this case the sub-phase of the LB trough contains Ln(III) ions and the amphiphilic free ligands are deposited on the sub-phase to complex with the Ln(III) ions at the air water interface. The last example (which will not be discussed in this review due to space limitations) involves ion-pair systems where ionic Ln(III) complexes contain amphiphilic counter-ions (e.g. anionic or cationic surfactants outside of the Ln(III) coordination sphere) [7, 8]. Again, due to the need for brevity, this review does not discuss the work on Langmuir-Blodgett films of Ln(III) bisphthalocyanines complexes, as this body of work has been thoroughly reviewed by Rodríguez-Mendez in 2009 and, to the best of our knowledge, there have been no reports of such systems since then .
Effect of film formation on Ln(III) emission
With sensing applications in mind, it is important to determine what effects (if any) the arrangement of Ln(III) ions in an ordered LB film has on the physical properties (i.e. emission properties) of the complex. The LB technique results in high local concentrations of amphiphiles in close proximity to a surface, therefore for Ln(III) containing films the biggest concern, especially if they are to be used as a sensor, is quenching of emission. A small number of studies have been carried out that investigated how film formation effected emission properties of the Ln(III) ions within the film.
Ln(III) Langmuir-Blodgett film sensors
Whilst many potential applications of Ln(III) based LB films have been proposed, one application that has begun to be realised is the ability of LB films to act as sensors. The previous sections have shown that LB films of amphiphilic Ln(III) containing complexes can be obtained relatively readily and such films are reasonably homogenous in coverage with deposition that does not always adversely affect photophysical output (i.e. Ln(III) luminescence). In the following section we will explore the small number of examples that are present in the literature where these types of surfaces act as sensors.
Dutton and Conte reported LB films of octafunctionalised calixresorcinarenes 13 and 14 (Fig. 6) which upon exposure to solutions of TbCl3 (2 × 10−4 M) abstract Tb(III) from solution, essentially acting as ion sequestration agents which respond to their local environment. This was an extremely important result as it showed that the formation of highly ordered LB films does not block the sensing component to modification from external perturbation, thus making LB films ideal for sensing . However, no comment on film stability upon repeated dipping was given.
Conclusions and future perspective
In this very brief mini-review, we have attempted to highlight the small number of LB films constructed from amphiphilic lanthanide complexes, in which at least one of the complexing ligands contains a covalently bonded amphiphilic moiety. Of the small family of Ln(III) amphiphilic systems made from both simple (e.g. 1–5, 19, 20) and complex (e.g. 6–18) ligands the film forming abilities have been studied in detail. This has led to an understanding of the fundamental affect/s that the lanthanide cations have on the LB films and the effect that the LB film environment has on the properties (luminescence) of the Ln(III) cations. Despite an understanding of fundamental properties, the application of these systems for advanced materials (e.g. surface bound sensors, molecular logic gates/molecular electronics) is still in its infancy. Given the retention of Ln(III) emission and good film coverage afforded by the LB method combined with initial sensing studies, the future of amphiphilic Ln(III) systems immobilised as LB films will no doubt be rich.
X-ray standing wave
circularly polarised luminescence
JAK conceived the idea for the review. Both authors read and approved the final manuscript.
The authors are grateful for the support of the Directed Assembly Grand Challenge Network. The authors also wish to thank Dr. Kelly Kilpin for helpful discussions and the University of Southampton for support of this work.
The authors declare that they have no competing interests.
The authors thank the Engineering and Physical Science Research Council for funding through grant references EP/N009185/1 and EP/K014382/1.
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