This project involves the development of novel sensory systems and arrays for the selective detection of ions and molecules (guests). This will be achieved by immobilising lanthanide based chemosensors (hosts) onto molecular defined surfaces and extend this to Atomic Force Microscope (AFM) probe tips using self-assembly monolayers (SAM) methodologies. We will evaluate both the photophysical properties of the recognition event as well as the nature of the host-guest interactions using AFM. This approach is of potential value for screening samples or mixtures of biological important electrolytes as well as small and medium size biomolecules such as single stranded DNA, proteins, etc.        

Development of luminescent signalling devices is an active area of research within supramolecular chemistry (1). Of particular interest to us are systems where the emission is modulated by single or multiple external sources (inputs) such as light, ions and molecules (2). In this regard luminescent switches, sensors, logic gate mimics, etc. have recently been reported (2, 3). Often, these systems employ fluorescence, emitting in the ns-range, whereas more recently, metal ion complexes and conjugates, that emit in the sub-ms range, have been developed (4). The latter has a particular advantage over the former for on-line and real-time chemosensing of biological samples as their long excited state lifetimes overcome auto-fluorescence and short lived light scattering in biological samples.

The use of lanthanide luminescence is particularly attractive for real-time, on-line sensing/screening application as these ions possess long excited-state lifetimes, emit in the red or near infrared, with characteristic line-like emission bands and large Stokes-shifts. The Gunnlaugsson research group, has extensively investigated the use of such lanthanide luminescent devices for sensing of cations, anions and neutral molecules in solution (5). However, for practical purposes it is desirable that such systems are embedded into membranes, such as hydrogels, or onto solid surfaces. The group has already initiated the development of such devices in soft materials by non-covalently incorporating a luminescent lanthanide pH-sensor into poly[methylmethacrylate-co-2-(hydroxyethyl-methacrylate)] based hydrogels and have investigated the luminescence properties of the resulting gels using both steady state fluorescence and confocal fluorescence spectroscopy (6). However, the incorporation of such lanthanide complexes onto solid substrates in a reproducible and well-defined manner has, to the best of our knowledge, not been previously demonstrated and we aim to address this in the proposed work.  

Our research plan aims at exploring the use of such lanthanide luminescent sensors (Ln-sensors) on surfaces, and the use of luminescence and AFM for screening biological samples. The Jarvis research group, has developed novel methods to probe the interactions of macromolecular structures on surfaces (8). We foresee that this mode of sensing could lead to the formation of multi-components sensory arrays where single molecules can be detected separately, or in the combination with both luminescent and scanning probe microscopy methods. Here the former will probe the presence of the target while the latter will give information about the resulting interactions between the target species and the solid bound sensors.

The use of AFM to detect binding events on a molecular level is well established (8). The initial aim of our work is to assemble a chemically defined surface using self-assembly that can detect or sense target analyte. Our ultimate aim is to combine on a single surface a series of sensors (hosts), thus expanding these novel sensory systems into multi sensory arrays. This is particularly attractive approach, as it would provide a platform for the screening of mixtures of biological samples, such as protein surfaces, antibodies, oligonucleotides, etc. To achieve this, we aim to absorb a single sensor (and later a patterned mixture of such sensors) onto surfaces such as gold, glass or oxides, or by non-covalently absorbing them on graphite or carbon nanotubes. To achieve this, we will design and synthesis several Ln-sensors similar to those previously developed in the Gunnlaugsson group. These will be modified by covalent linkers such as butanethiol or thioactic acid that provide the covalent attachment of these sensors to gold surfaces. Alternatively, these can be incorporated using other functional groups such as silicon oxides, for the attachments onto glass, or by using acetates with the aim of using metal oxides. The attachment of the complexes can also be achieved by furnishing the surface with appropriate linkers and then covalently attaching the sensor to it. This could potentially give more precise control of the surface density and distribution of the sensors can be better controlled. 

The use of luminescent spectroscopy for the characterisation of SAMs modified surfaces has been limited until now because gold surfaces can quench the excited state of fluorophores; secondly, potential quenching can also occur by nearby molecules. Whereas the former can be overcome by using different types of surfaces, such as glass, the latter is function of the sensor loading, nature of the linker employed and the composition of the monolayers. Optimising these conditions to yield a surface with maximum sensor density, where the receptor molecules are properly orientated, while maintaining minimal steric constraints and crowding, is significant challenge that we hope to address in this work. The orientation of the sensor at the surface has been shown to affect the efficiency of the surface binding, therefore the ability to have uniform distribution of the sensors orientation will enable higher binding efficiency. We foresee that substantial research time will be devoted to optimise these conditions and investigating the luminescence respond from the SAMs for our sensors. In addition to this we will use AFM to provide a range of fundamental information on the structure of the sensor interface, allowing these systems to be optimised in a systematic way.

The use of lanthanide complexes is particularly attractive for developing sensory arrays. As ions such as Tb(III), Eu(III) emit in the green and red part of the electronic spectrum respectively, other ions such as Yb(III) and Nd(III) emit in the near infrared. As it is our aim to furnish different sensors with different lanthanide ions, we hope to be developing SAMs that emit at different wavelengths upon excitation by a single wavelength. Hence, a particular host:guest interaction will give rise to a particular emission wavelength, and the resulting surface area can be probed using AFM. From these combined techniques, both the sensitivity and the selectivity of the sensing as well as structural information can be obtained.

[1] e.g. A.P. de Silva, H.Q.N. Gunaratne, T. Gunnlaugsson, A. J. M. Huxley, C. P. McCoy, J. T. Rademacher and T. E. Rice, Chem. Rev., 1997, 97, 1515.

[2] F. M. Raymo, Adv. Mater. 2002, 14, 401; A.P. de Silva, et al, Coord.Chem. Rev., 2000, 205, 41; T. Gunnlaugsson, et al, J. Am. Chem. Soc., 2001, 123, 12866; T. Gunnlaugsson, et al, Chem. Commun., 2000, 93.

[3] D. Parker and J. A. G. Williams, J. Chem. Soc., Dalton Trans., 1996, 3613; D. Parker, Coord. Chem. Rev., 2000, 205, 109.

[4] D. Parker, et al, Chem. Rev. 2002, 102, 1977.

[5] e.g. T. Gunnlaugsson, et al, Chem. Commun., 2004, 782; T. Gunnlaugsson and J. P. Leonard, Chem. Commun, 2003, 2424; T. Gunnlaugsson, et al, J. Am. Chem. Soc. 2003, 125, 12062; T. Gunnlaugsson, et al, Supramol. Chem. 2003, 15, 505; T. Gunnlaugsson, et al Chem. Commun., 2002, 2134; T. Gunnlaugsson, Tetrahedron Lett. 2001, 42, 8901.

[6] T. Gunnlaugsson, C. P. McCoy, R. J. Morrow, C. Phelan and F. Stomeo, Arkivoc, 2003, 8, 216.

[7] e.g. M. A. Lantz, S. P. Jarvis, H. Tokumoto, T. Martynski, T. Kusumi, C. Nakamura and J. Miyake, Chem. Phys. Lett., 315, 61 (1999); S. P. Jarvis, et al, J. Phys. Chem. B, 26, 6091 (2000).

[8] E.-L. Floin, et al, Science, 1994, 264, 415; G. U. Lee, et al, Science, 1994, 266, 771.