Chemical Sensing Based On Multi-component Self-assembly

Host-guest recognition and molecular sensing have been important research areas.The traditional approach is to design the host with a binding site that matches the size and shape of the guest based on the implementation of "lock and key" principle.However,this often requires the design of complicated structures and lengthy synthesis.New approaches have been developed in the concept of adaptive self-assembly and dynamic covalent chemistry.By using reversible interactions such as hydrogen bonding,hydrophobic interaction,aromatic stacking,metal coordination,electrostatic interaction or dynamic covalent bonds,multiple components can self-assemble into a supramolecular host that can achieve guest binding with high affinity and selectivity that rival the synthetic counterparts while minimizing synthetic efforts.Supramolecular polymers are a class of molecular assemblies that are promising platforms for molecular recognition and sensing.The polymerization process increases the local concentration of host binding sites,which can potentially enhance the binding affinity between the host and guest and improving the sensitivity when using these assemblies as sensors.Multivalent interactions between supramolecular polymers and guest molecules can lead to high selecti vity for the guest of interest.This dissertation consists of five chapters.Chapter 1 gives an overview of different research subjects relevant to work in this thesis,including(a)the application of imine bond and boronate ester chemistry in the self-assembly,(b)perylenebisimide aggregates,(c)molecular recognition and sensing based on supramolecular aggregates,(d)differential sensing and(e)chirality sensing.In Chapter 2,a combination of boronic acid functionalized perylenebismide(PBI)PBIProBA and aldehyde functionalized PBI PBIEtpAld was employed as a sensing ensemble for the neurotransmitter L-DOPA.Taking advantage of PBI aggregation and the synergistic effect of various weak interactions,highly sensitive and selective sensing of L-DOPA at physiological pH has been achieved.It was proposed that the PBI aggregates provide a hydrophobic microenvironment to stabilize the water-susceptible imine bonds.L-DOPA binding led to the formation of optically active aggregates showing induced circular dichroism(CD)signals from the achiral perylene chromophore.The induced CD signals at 500 nm allows chiral sensing of DOPA in the visible region,eliminating potential interference of species that absorb in the ultraviolet region.In Chapter 3,a chiral sensing system for amino acids was developed based on aggregation of a guanidinium-functionalized PBI,PBIEtGua,induced by a diimine adduct formed from a bisaldehyde and amino acids.A bisaldehyde was used as a linker to join two molecules of amino acids via imine condensation to generate a diimine adduct that contains two carboxylate groups.The diimine adduct cross-linked PBIEtGua dyes via guanidinium-carboxylate interactions thus promoting aggregation of PBIEtGua.By linear discrimination analysis of the CD data of the assemblies obtained using different bisaldehydes in different solvents,17 common amino acids were successfully distinguished from each other and the unknown amino acid species could be identified.Previous work by our group have demonstrated an unusual CD-ee relationship opposite to the "majority rules" effect,in malic acid-induced aggregation of PBIEtpBA,which was termed"anti-majority rules" effect.Such an effect was characterized by the observation of the steepest slope around 100%ee when plotting the CD signal as a function of ee.This feature is promising for the determination of enantio-compostion of chiral organic compounds in the high ee region.In Chapter 4 the applicability of "anti-majority" rules-operative systems in sensing ee close to 100%was examined and the possibility of rational designing such systems was explored.The accuracy of ee determination in three selected systems that exhibit linear CD-ee relationship,"majority rules" and "anti-majority rules",respectively,was compared.Among the three systems,the smallest relative error was found with the "anti-majority" rules-operative system.We hypothesized that the "anti-majority" rules effect originates from the preferential binding of the host to racemic mixtures of the guest that result in the formation of meso-adducts.Based on this concept,we explored the bisaldehyde-amino acid-PBIEtGua system due to the ability of the bisaldehyde to form meso-adducts with a mixture of amino acid enantiomers of varying ee.In Chapter 5,an in situ formed fluorescent bisboronic acid sensor was used for selective sensing of glucose.The three-component assembly of o-phthalaldehyde,aminomethylbenzeneboronic acid and mercaptophenylboronic acid led to the formation of a fluorescent isoindole compound that contains two boronic acid functionalities,which can form cyclic boronate esters with glucose that contains two boronic acid binding sites.When the isoindole was pre-formed in MeOH and used as a sensor in water,a selective fluorescence enhancement response to glucose was be observed presumably due to the restriction of the fluorophore conformation.Interestingly when the three components were mixed in situ at low concentrations in water,the reaction rate and yield for the isoindole formation was saccharide-dependent where glucose but not fructose was found to significantly promote the isoindole formation.