Hydrogen-bonded Assemblies Containing Dopamine:Construction And Stimuli-responsive Behavior

Inspired by the excellent properties of biological species in nature,the construction of smart materials has attracted lots of attentions.As a structure mimic of mussel adhesive proteins,dopamine can deposit on virtually any material surface.The one-step deposition of dopamine has been developed into a strong strategy for the construction of functional materials.Furthermore,dopamine has a series of characteristic chemical reactions,which could be utilized for the post-functionalization.It opens a new insight for the fabrication of smart materials.Moreover,layer-by-layer assembly technology is a versatile and simple method for the fabrication of materials with well-controlled thickness and tunable functionality.Lots of functional materials with pH responsiveness have been constructed through hydrogen-bonding layer-by-layer assembly.Hydrogen-bonded layer-by-layer materials have shown potential applications in drug delivery and antifouling coating.In this thesis,stimuli-responsive free-standing thin film,Janus film and complex microcapsules have been successfully fabricated by taking the advantages of one-step deposition of dopamine,dopamine chemistry and hydrogen bonding layer-by-layer assembly.Furthermore,their properties and potential applications have been studied.(1)Dopamine-modified poly(acrylic acid)(PAA-dopa)and poly(vinylpyrrolidone)(PVPON)was layer-by-layer(LbL)assembled to prepare PVPON/PAA-dopa thin film based on hydrogen bonding.Furthermore,the effect of assembly pH value and Molecular weight on the assembly behavior of PVPON/PAA-dopa thin film was studied.During the assembly process of PVPON/PAA-dopa thin film,PVPON and PAA-dopa acted as hydrogen bond acceptor and donor,respectively.The carboxylic group of acrylic acid and the phenolic hydroxyl group of dopamine could both form hydrogen bond with the carbonyl group of PVPON.There is relatively strong intra-molecular hydrogen bonding existed in PAA-dopa chain.At low pH conditions,PAA-dopa would aggregate,thus could not be layer-by-layer assembled with PVPON.At pH 3.0,PAA-dopa is in aggregated state and well dispersed in the solution,it could form multilayer with PVPON via hydrogen bonding.With the increase of pH value,PAA-dopa protonated and hydrogen bonds were weakened,and hence the thickness of film decreased.Because of the high pKa of dopamine,there are hydrogen bonds formed between PVPON and PAA-dopa at a relatively high pH condition.At pH 3.0-4.5,PVPON/PAA-dopa film could be successfully fabricated.The critical assembly pH value of PVPON/PAA-dopa film is enhanced compared with PVPON/PAA film.Subsequently,the influence of molecular weight on the assembly behavior of PVPON/PAA-dopa thin film was studied.At pH 3.0,PVPON(Mw = 40 000)was used as hydrogen bond acceptor,and layer-by-layer assembled with PAA-dopa with different molecular weights.PAA-dopa(PAA,Mw = 100 000)with relatively low molecular weight was assembled at the expanded chain conformation,thus a relatively smooth thin film was formed.With the molecular weight increase,PAA-dopa(PAA,Mw = 450 000)exhibited strong intra-molecular hydrogen bonding,it was assembled at the compact conformation.Thus,the film was rough and had a relatively high thickness.On the other hand,at pH 3.0,PAA-dopa(PAA,Mw = 100 000)was used as hydrogen bond donor,and layer-by-layer assembled with PVPON with different molecular weights(Mw = 10 000,40 000,360 000,1300 000).And all the films showed regular and smooth surface morphologies.With the molecular weight higher than 40 000,the mobility of the PVPON chain declines,the thickness decreased gradually.(2)The stability of PVPON(Mw = 40 000)/PAA-dope(PAA,MW = 450 000)film assembled at pH 3.0 was investigated.Furthermore,dopamine chemistry was utilized for the cross-linking of the PVPON/PAA-dopa film.The free-standing PVPON/PAA-dopa film was fabricated with the pH-responsive swelling/shrinking behavior.PVPON/PAA-dopa film was incubated in HCl and NaOH solution with different pH values.The film was stable in the weak acid solution,and it would dissociate at pH higher than 7.0.The critical disintegration pH value of PVPON/PAA-dopa film is enhanced compared with PVPON/PAA film.For further improving the stability of hydrogen-bonded PVPON/PAA-dopa film,catechol chemistry of dopamine was utilized to cross-link the thin film.Sodium periodate(NaIO4)was chosen as oxidant,dismutation reaction took place between the catechol groups.As the reaction condition was mild,no obvious morphology change occured after cross-linking.By immersing the film in alkaline solution,the hydrogen bond between the bottomlayer PVPON and the substrate broke,thus the film could be exfoliated from the substrate to get a free-standing film.Moreover,by tuning the environmental pH value,the carboxylic group of PAA-dopa would deprotonation or protonation,resulting in the hydrogen bond in the film break or reconstruct,thus the free-standing film showed reversible swelling-shrinking behavior.After the hydrogen bond broken,PAA-dopa chain transformed from a compact state to an expanded state gradually,and the original aggregate structre was disappeared.With the hydrogen bond rebuilt in acid solution,PAA-dopa chains rearranged and stacked regularly with PVPON.Therefore,the reformed compact film exhibited a smooth surface morphology.pH-responsive PVPON/PAA-dopa free-standing film shows potential application in biosensing area.(3)By combining the one-step deposition of dopamine with layer-by-layer assembly,(PAA-dopa/PVPON)nPDA complex film was successfully fabricated.And it could be used as a platform for the construction of asymmetric Janus film.Through multi-step post-modification,pH-responsive(PAA-dopa/PVPON)nPDA/Ag/PFDT Janus film was fabricated,which exhibit anisotropic superhydrophobic/hydrophilic surfaces and asymmetric surface-enhanced Raman scattering performance.Firstly,(PVPON/PAA-dopa)20 film was layer-by-layer assembled via hydrogen-bonding at pH 3.0.Then,polydopamine(PDA)was deposited on the multilayer through the oxidized polymerization.(PVPON/PAA-dopa)20 hydrogen-bonded film is unstable at pH higher than 7.0 and will dissociate gradually.Thus,PDA deposition was performed in the neutral environment by the addition of NaIO4(oxidant).During the deposition of PDA,robust(PVPON/PAA-dopa)nPDA film was fabricated with the cross-linking structure via dismutation reaction of catechol groups.Further,PDA has various unique properties,such as reducibility,self-adhesive nature,and post-modificationability.Silver ions could be situ-reduced on the(PVPON/PAA-dopa)nPDA film and form silver nanoparticles.The film could be further modified with PFDT via Michael addition.By immersing in basic solution,(PVPON/PAA-dopa)nPDA and post-modified films could be exploited from the substrate,and form free-standing thin films with pH-responsive behavior.(PVPON/PAA-dopa)nPDA/Ag/PFDT free-standing film showed asymmetric wettability,superhydrophobicity and hydrophility.Moreover,it showed asymmetric surface-enhanced Raman scattering(SERS)performance,which could be used for trace detection.By combining the powerful dopamine chemistry with hydrogen-bonded LbL thin film,(PVPON/PAA-dopa)nPDA complex film was fabricated.This complex film could be utilized as a platform for the construction of advanced Janus membranes with pH responsiveness.These Janus thin films show potential applications in soft sensor and separating membrane.(4)PDA(PAA-dopa/PVPON)n complex capsules were fabricated by combining the polymerization of dopamine with hydrogen-bonding layer-by-layer assembly(LbL)of dopamine-modified poly(acrylic acid)(PAA-dopa)/poly(vinylpyrrolidone)(PVPON).PDA(PAA-dopa/PVPON)n complex capsules showed pH-responsive swelling—shrinking behavior.Furthermore,it realized controlled loading and release of model dye with the change of environmental pH value.Polydopamine(PDA)was firstly deposited on the SiO2 particles,then PAA-dopa and PVPON were alternatively assembled on the PDA-coated particles.Through the dismutation reaction among the catechol groups of PDA and PAA-dopa,PDA(PAA-dopa/PVPON)n shell was cross-linked.The complex capsule was fabricated by etching the template particle.During LbL assembly,PAA-dopa and PVPON penetrated to the robust PDA pre-layer,thus the complex capsules became soft and flexible.Moreover,PDA capsule exhibited a rough surface,after modified with PAA-dopa/PVPON,the surface became relatively smooth.With the change of environmental pH value,the hydrogen bond between PAA-dopa and PVPON would dissociate or reconstruct,thus PDA(PAA-dopa/PVPON)n capsules exhibited pH-responsive swelling—shrinking behavior.The method gives PAA-dopa/PVPON layer a robust holder and endues PDA layer with pH responsiveness.With only one additional PAA-dopa/PVPON bilayer,PDA(PAA-dopa/PVPON)i capsules showed a volume swelling ratio of 1.34 from pH 2.0 to pH 8.5.This strategy simplifies the fabrication of pH-responsive LbL capsules.Meanwhile,it restrains the particle aggregate caused by the multi-step centrifugation/redispersion cycles,leading to the enhancement of production efficiency.In addition,the complex capsules exhibited controlled loading and release of Rhodamine 6G,which could be considered as a potential candidate for drug delivery and biosensor.