PET sensors are very simple in principle and can signal the presence or absence of a very small amount of analyte in solutions/biological samples by using a readily available instrument – the fluorimeter. This article looks at the design principles of PET sensors, as well as examining how they may be modified to enhance specificity/selectivity. [Aug 2009]
Luminescent sensors provide for an easy, often visual, method for detection of a wide range of ions, physical properties such as pH and other components such as nanoparticles. Their wide variety of sensing capabilities are already used in commercial devices, and a greater understanding of their mechanism of operation is leading to newer exciting concepts, such as molecular logic gates for computing. This article aims to summarise the background theory to luminescent sensors – especially concentrating on photoinduced electron transfer (PET) which we can consider as the third type of quenching (Type I and Type 2 discussed elsewhere – links will be here) – and examining practical applications in the context of this theory. In summarising the work here, special attention is paid to the work of Prof A. P. de Silva who is based in Queen’s University, Belfast and who is one of the pioneers in the area. Interested students are referred to some of his papers in the references, where even in the formal constraints of academic journals, his passion for the subject is very evident. Here’s a short podcast outlining what we will cover in this section (with audio):
We’ve seen several times that molecules that have absorbed light may transfer their electron to another system – which is usually called a quencher, although is not constrained to this (see for example dye-sensitized solar cells). Because this electron is transferred after absorption of light by the molecule, the process is called photo-induced electron transfer (PET). It’s one of the great applications of photochemistry in general – excited states are better oxidisers and reducers than their ground state counterparts. Suppose we have donor (reducing agent, itself going to be oxidised) and acceptor (oxidising agent, itself going to be reduced) molecules as shown below, where the energy level of the donor HOMO is lower than the energy of the acceptor LUMO. As it stands, the donor cannot reduce the acceptor, as electron transfer is not energetically feasible. However, if we form a donor excited state, D*, by light excitation, the process becomes energetically feasible once the donor LUMO is higher than the acceptor LUMO. You should consider how excited states are also better oxidising agents.
In applying this general concept to this section, we will be looking at molecules that have both these components in one molecule. In this system, we will term them luminophore and receptor, which are analogous to the discussion of donor and acceptor respectively. The luminophore is the light absorber which results in an excited state and may luminesce, or accept an electron into the vacancy in the ground state, or donate an electron from the excited state. The receptor will accept/donate an electron from/to the luminophore. The two components are linked by a non-conjugated bridge (e.g. two sp3 carbon bridge). You may wish to consider why a non-conjugated bridge is required. The three components – luminophore-spacer-receptor – make up what we will term here a class III type of quenching; where the PET process occurs within one molecule.
The power of the model is that if the receptor’s energy level’s can be tweaked in the presence or absence of an analyte – for example an metal cation – and the energy levels between the receptor and luminophore are close, well the presence of the ion may turn on or shut off the electron transfer process. This concept is now a sensor, as luminescence is affected by the presence or absence of an analyte. If that luminescence can be measured (which it can), we have a very powerful, sensitive analytical tool to measure the extent of analyte present.
The general principle discussed above is summarised in the diagram below. A luminophore gives off light in the absence of an analyte, but emission is quenched in the presence of an analyte. This is called an ON-OFF system – emission was turned off by the presence of an analyte. The systems become very powerful when the receptor – the docking area for the analyte – is selective to only a particular type of species (e.g. a halide) or even better a specific ion. An example of one of de Silva’s systems is shown below. The three components are obvious: the anthracene molecule is the luminophore (in this case a fluorophore as it emits from the excited singlet state); the bipyridyl component is the receptor and the two carbon-chain acts as a spacer. In the absence of an ion, this system fluoresces strongly. However, in the presence of either zinc ion of a proton the emission is shut off. So how can we explain what happened?
Examining the emission spectrum, we can see that the anthracene emission is very much reduced in the presence of one of the specific ions, but the shape or position of the bands do not change. Therefore its energy levels remain as they were prior to the presence of the receptor. It can be concluded that the receptor levels change, opening up a PET pathway that was not available prior to the presence of the analyte. The diagram below summarises the schematic, molecular and energetic processes.
3.2 Developing the idea further
In a paper discussing the developments of luminescent sensors so far this century, de Silva illustrates a scenario that is already a reality (Tetrahedron, 2005, 61, 8551 – 8588). An ambulance is called for a car accident trauma, and upon arrival paramedics take a blood sample. From this, they can alert the hospital en route of the required electrolyte levels required for a blood bag, having used a small portable instrument to determine concentrations of for example, sodium, potassium, calcium as well as oxygen levels and pH. The key here is that the the device incorporates a range of PET sensors, each of which have receptor components of the moleculethat selectively sense a particular of ion/molecule. Therefore, the question is how is this done?
Selectivity has been achieved by a range of interesting approaches, and we have to be thankful to organic chemists for their synthesis of the diverse range of molecules available. Crown ethers are useful receptors, and are known to bind well to sodium ions. The image below is from de Silva’s homepage. It shows the use of a crown ether in a PET sensor that selectively senses for sodium. The principle is the same as above (except this is now an OFF-ON sensor – you should sketch out the energy levels to see how emission is turned on in the presence of sodium ions)
Now that the general principle is understood, it is easy to get selective across a wide range of ions. By modifying the size of the crown ether, the sensor can lose its selectivity for sodium ions and move on to potassium ions (see image below). Various minor modifications can make it calcium selective. Very quicky, the range of ion selectivities can be acquired.
The principle can also be applied as a pH monitor, by developing a proton sensitive detector. In this case, amines linked to an anthracene molecule via a spacer make for a very simple but effective pH sensor. An example is shown below – a clever attribute of this design is that the use of two amines mean that much greater sensitivity across the range of pH is “built in” to the molecules design. The emission at various pH values is shown alongside the molecule. You should aim to sketch out the appropriate changes to the molecule on increasing acidity resulting in this change in emission. As well as simple pH monitors, these types of materials have been used in examining the effectiveness of cancer treatment. Cancer cells respond to treatment by developing acidic environments, and by examining the emission of a pH sensor on treating with radiation. The images, available in fig. 3 in the original paper show increased pH, easily detected, indicating that treatment is having an effect.
As well as cations, a lot of work has been invested into developing anion sensors. Gunnlaugsson, based in Trinity College Dublin is a leading researcher in this area and a suummary of many of his results can be found in the reference at the end. A video demonstrating the effect will be posted here September 09.
The principle of photo-induced electron transfer can be utilised in the development of molecular sensors. The mechanism of PET was examined above, as well as looking how shutting off or turning on this process provides information on the nature of ions interacting with the sensor. Sensors can be designed so that they selectively sense an ion in the presence of others. Current research in this area is looking at using these principles in molecular computing, and this work will be surveyed in a future article.
[Aug 2009 - updates/amendments will be logged in comments.]
Newer optical-based molecular devices from older coordination chemistry, de Silva, A. P., McCaughan, B., McKinney, B. O. F. and Querol, M, Dalton Trans., 2003, 1902 – 1913. Really excellent overview of the area with lots of examples.
Luminescent sensors and switches in the early 21st century, Callan, J. F., de Silva, A. P. and Magri, D. C., Tetrahedron, 61, 8551 – 8588. Overview of developments in 21st Century across range of analyte types (cations, anions, molecules)
Anion recognition and sensing in organic and aqueous media using luminescent and colorimetric sensors, Gunnlaugsson, T., Glynn, M., Tocci (nee Hussey), G. M., Kruger, P. E. and Pfeffer, F. M., Coord. Chem. Rev. , 2006, 250, 3094–3117. Good overview of anion sensors.