Regulation of aggregation and morphology of cyanine dyes on monolayers via gemini amphiphiles

The aggregation of dyes is of considerable importance in relating to their functions and applications. In this paper, we have investigated the regulation on the aggregation and morphology of two cyanine dyes, 3,3'-disulfopropyl-4,5,4',5'-dibenzo-9-methylthiacarbocyanine triethylammonium salt (MTC) and 3,3'-disulfopropyl-4,5,4',5'-dibenzo-9-phenylthiacarbocyanine triethylammonium salt (PTC), using a series of gemini amphiphiles (bis(2'-heptadecyl-3'-ethylimidazolium)-1,n-alkane dibromide, abbreviated as Gn, n=2, 4, 6, 8, 10). It has been found that both of the dyes could be adsorbed onto the monolayers of the gemini amphiphiles through the electrostatic and pi-pi interaction and stacked into H- or J-aggregate. The spacer of the gemini amphiphile showed good control over the aggregation of MTC: H-aggregate was favored when gemini amphiphiles with short spacer were applied, while J-aggregation was preferred in the case of longer spacer. Only J-aggregate was observed for PTC on gemini monolayer, regardless of the structure of the gemini amphiphiles. Interesting morphologies were observed for all the gemini/dye complex monolayers. Network structure and nanofibers were formed for the gemini/MTC films transferred below the plateau surface pressure and close to the collapse pressure, respectively. The ability of the complex monolayers to form nanofibers strongly depended on the component amphiphiles, G2>G4>G6, and no nanofibers were observed for G8/MTC and G10/MTC after the collapse. Only squared domains were observed for gemini/PTC monolayers. When both G2 and G10 were mixed, an individual control of the gemini amphiphiles over the aggregation of MTC in the complex monolayers was observed. The relationship among the spacer, dye structure, and aggregation was revealed.     

Classification for molecular aggregation structure of monolayers on the water surface

The melting temperature, T_m, and the crystalline relaxation temperature, Tα_c, of palmitic acid and dipalmitoyl phosphatidylcholine monolayers on the water surface were evaluated by combination of two kinds of measurements: first, the subphase temperature, T_sp, dependence of the monolayer modulus based on the surface pressure-area (π-A) isotherm and second, the T_sp dependence of the electron diffraction, ED patterns of their monolayers. On the basis of their characteristic temperatures of the monolayers, the aggregation structure of the monolayers which were transferred onto a hydrophilic SiO substrate at various surface pressures and T_sps was investigated by means of transmission electron microscopy. The π-A isotherm for the fatty acid monolayer on the pure water surface represented the aggregating process of isolated domains grown right after spreading a solution on the pure water surface. The fatty acid monolayer on the pure water surface was classified into a crystalline monolayer (T_sp < T_m) and an amorphous one (T_sp > T_m). The crystalline monolayer was further classified into two types; crystalline domains were aligned along their crystallographic axes owing to an induced sintering at the interfacial region among monolayer domains by surface compression (T_sp < Tα_c), while not for T_sp > Tα_c. In the case of the phospholipid monolayer, the monolayer was classified into a compressing crystallized monolayer (T_sp < T_m) and an amorphous one (T_sp > T_m). The compressing crystallized monolayer is a monolayer in which crystallization was gradually induced at plateau region on the π-A isotherm by compression. Electron diffraction studies of arachidic acid monolayers in different dissociated states of hydrophilic groups revealed that formation of the compressing crystallized monolayer was attributed to an electrostatic repulsion among ionic hydrophilic groups. It was concluded that the aggregation structure of monolayers on the water surface was systematically classified into ‘the crystalline monolayer’, ‘the amorphous monolayer’ and ‘the compressing crystallized monolayer’, with respect to thermal and chemical (intermolecular repulsive) factors.     

Theoretical description of aggregation of cationic gemini surfactants in the bulk solution and on the silica surface

A theory of cationic dimeric (gemini) surfactant adsorption onto negatively charged surface is presented. In the proposed model it is assumed that the adsorbed phase is a mixture of singly dispersed molecules of surfactant and spherical, globular and cylindrical aggregates of different dimensions. Only the “excluded area” interactions between the adsorbed species are considered and the effects of surface heterogeneity on monomer adsorption are taken into account. The aggregation behavior of gemini surfactants is based on the additive free energy model proposed by Camesano and Nagarajan (2000). The calculated surfactant adsorption isotherms and the differential molar enthalpies of micellisation and adsorption are compared with the experimental results obtained for a series of gemini surfactants depending on the length of a spacer, temperature or the presence of electrolyte. On the basis of theoretical results the evolution of adsorbed phase of gemini surfactants with the increasing adsorption is discussed. It is shown that the evaluated cmc values and the dimensions of surfactant aggregates are in a good agreement with experiment. Unfortunately, the theoretical model does not describe properly the temperature dependence of micellisation process.     

Effect of polyelectrolytes on aggregation of a cyanine dye

The effect of poly(acrylic acid) and poly(methacrylic acid) on the molecular state of a cationic cyanine dye (3,3′-diethyl-2,2′-thiacarbocyanine iodide) was studied. Gels based on the above polymers were shown to efficiently absorb an oppositely charged dye. The absorption of dye ions by a gel induced their aggregation. It was shown that, in the acrylic acid-based gels, H-aggregates, dimers, and single ions of the dye coexist. In gels containing methacrylic acid units, dye ions mainly formed H-aggregates. A comparison of aggregation of dye ions in the gels with that in aqueous solutions of corresponding polyelectrolytes was performed. When dye ions were immobilized in a gel network, the proportion of the ions forming the H-aggregates was considerably larger than that in solutions. The effect of the gel network charge density on the aggregation of dye ions was investigated. It was shown that an increase in the fraction of charged units in network chains facilitates aggregation.     

Study on the adsorption and aggregation of dilute gemini surfactant solutions

As a function of temperature in aqueous solutions, the adsorption and aggregation of N,N’-bis (tetradecyl dimethyl)-1,2-dibromide-ethanediyl ammonium salt (GS14-2-14) and N,N’-bis (hexadecyl dimethyl)-1,2-dibromide-ethanediyl ammonium salt (GS16-2-16), were researched with drop-volume technique and conductometry, respectively. The results of surface tension measurements, which were analyzed by originally developed thermodynamic equations, illustrate that GS14-2-14 has a better surface activity and arranges more tightly in the adsorbed film than GS16-2-16. The data of conductivity were used to find critical micelle concentration (cmc) and counterion binding degree of micelle (β). Thermodynamic parameters of micellization were also obtained from the temperature dependence of cmc values. From the study, it is discovered that the micellization process is spontaneous and exothermic in nature and it is mainly driven by entropy.     

新型腰果酚双子表面活性剂的合成及表面活性

以天然生物质腰果酚、1,3-二溴丙烷及氯磺酸为原料,通过醚化、磺化及中和三步反应合成了一类新型的腰果酚基磺酸盐双子(Gemini)表面活性剂.采用傅立叶转换红外光谱仪和核磁共振谱仪表征了产物的结构;采用滴体积法测定了腰果酚Gemini表面活性剂的表面张力,研究了水溶液的表面性质,并与相应的单基腰果酚基磺酸盐表面活性剂进行了对比.结果表明:腰果酚Gemini表面活性剂水溶液的临界胶束浓度(cmc)为6.20×10-2 mmol.L-1,远小于相应的单基腰果酚表面活性剂水溶液的cmc(8.40mmol.L-1);其临界表面张力γcmc为36.92mN.m-1,与单基腰果酚表面活性剂水溶液的相近(γcmc为38.41mN.m-1).与此同时,腰果酚Gemini表面活性剂水溶液的最小分子截面积Amin为0.27nm2,比相应的单基表面活性剂水溶液的小得多.     

Studies of the aggregation behavior of cyclic gemini surfactants

The specific conductance, surface tension, mean aggregation number, and apparent molar volume properties of aqueous solutions of a novel series of N,N'-bis(cyclododecyldimethyl)-alpha,omega-alkanediammonium dibromide (c12-s-c12) surfactants, where s is the spacer chain length, are reported. Surfactants with s = 3, 4, and 6 have been prepared and characterized in terms of their Krafft temperature (T(Kr)), critical micelle concentration (cmc), surfactant head group area (a) at the air-water interface, mean aggregation number (N(agg)), and the volume change upon micelle formation (deltaV(phi,M)). The c12-3-c12 shows little evidence of aggregate formation, while the results obtained for the c12-4-c12 and c12-6-c12 homologues suggest the formation of small, poorly defined micellar aggregates in aqueous solution.     

Effects of ethylene glycol addition on the aggregation and micellar growth of gemini surfactants

Micellization of three didodecyl dicationic dibromide gemini surfactants with different methylene spacer lengths, 12-s-12,2Br- where s = 3-5 methylene groups, has been investigated in water-ethylene glycol, EG, mixtures with weight percentages of EG up to 50%. Subsequently, effects of the addition of the organic solvent on the micellar growth of these surfactants and on the surfactant concentration range where sphere-to-rod transitions occur were studied by means of steady-state and time-resolved fluorescence quenching and spectroscopic measurements. Results show that an increase in the weight percentage of ethylene glycol added to aqueous 12-s-12,2Br- (s = 3-5) micellar solutions causes the sphere-to-rod transition to occur at higher surfactant concentrations than in pure water. The diminution in the average aggregation number, N(agg), when wt % EG increases, provoked by the decrease in the interfacial Gibbs energy contribution to DeltaG degrees M, is the main factor responsible for this observation. The decrease in N(agg) is accompanied by a decrease in the ionic interactions and the extra packing contribution to the deformation of the surfactants tails, making formation of cylindrical micelles less favorable. Besides, an increase in the solvent content and polarity of the interfacial region does not favor formation of direct ion pairs, decreasing the tendency of micelles to grow.     

Improved molecular rectification from self-assembled monolayers of a sterically hindered dye

Self-assembled monolayers (SAMs) formed from the reaction of 1-(10-acetylsulfanyldecyl)-4-[2-(4-dimethylaminonaphthalen-1-yl)-vinyl]-quinolinium iodide (1a) and gold-coated substrates exhibit asymmetric current-voltage (I-V) characteristics with a rectification ratio of 50-150 at +/-1 V. It is the highest to date for a molecular diode, and the improved behavior may be assigned in part to the controlled alignment of the donor-(pi-bridge)-acceptor moieties and in part to steric hindrance, which imposes a nonplanar structure and effectively isolates the molecular orbitals of the donor and acceptor end groups. The molecular origin of the rectification is verified by its suppression upon exposure to HCl vapor, which protonates the dimethylamino group and inhibits the electron-donating properties, with restoration upon exposure to NH3. It is also established by a reduced rectification ratio of ca. 2 at +/-1 V when the cationic D-pi-A+ moieties adopt an antiparallel arrangement in self-assembled films of the derivative, bis-[1-(10-decyl)-4-[2-(4-dimethylaminonaphthalen-1-yl)-vinyl]-quinolinium]-disulfide diiodide (1b), which adsorbs via one of its terminal donors without rupture of the sulfur-sulfur bond: Au/D-pi-A+-C10H20-S-S-C10H20-+A-pi-D (I-)2.     

Role of the spacer stereochemistry on the aggregation properties of cationic gemini surfactants

The preparation and characterization of three stereoisomeric cationic gemini surfactants, 2,3-dimethoxy-1,4-bis(N-hexadecyl-N,N-dimethylammonio)butane dibromide, are described. The aggregation properties have been studied by fluorescence, electrical conductivity, and quasi-elastic light scattering. A conformational study of the surfactant headgroups has been performed by molecular mechanics calculations to investigate the effect of the stereogenic centers on the surfactant molecular shape and therefore on the different organizations of the monomers in the aggregates. Results show that the stereochemistry of the spacer strongly influences the aggregation behavior of the diasteromeric gemini in water.     

Liquid crystal-directed assembly and phase morphology of a squaraine dye

The squaraine (SQ) dye, 2,4-bis [4-(N,N-di-n-hexylamino)-2-hydroxyphenyl]squaraine was synthesised, and a series of SQ dye/cholesteryl pelargonate binary mixtures with varying dye concentrations (1%, 3%, 7.5%, 10.8%, 15% and 20% w/w) were prepared. Their phase transitions were investigated using differential scanning calorimetry, polarised light microscopy and X-ray diffraction. The SQ dye itself exhibited no liquid crystalline behaviour. The concentration of the dye in the cholesteric compound proved to have a significant effect on the dye aggregation behaviour and phase transitions in cholesteryl pelargonate, resulting in the appearance of new mesophases and the formation of J- and H-aggregates. The texture morphology, X-ray diffraction data and UV–vis absorbance spectra provide strong evidence of the viability of liquid crystal-directed assembly of the SQ dye.     

Adsorption kinetics of p-nitrobenzenethiol self-assembled monolayers on a gold surface

The adsorption kinetics of octadecanethiol (ODT) and p-nitrobenzenethiol (NBT) from ethanol solutions has been studied by means of contact angle, optical ellipsometry, angle-resolved X-ray photoelectron spectroscopy (ARXPS), and grazing angle attenuated total reflection Fourier transform infrared (FTIR) measurements. ODT data were used as a reference for the kinetics studies of film growth. The growth of self-assembled monolayers from dilute solutions follows Langmuir isotherm adsorption kinetics. A saturated film is formed within 5 h after immersion in solutions of concentrations ranging from 0.0005 to 0.01 mM. The density of the monolayer depends on the concentration of the solution.     

Surfactant monolayers on a clean mercury surface: Apparatus and experiments

A Langmuir trough for studying monolayers on a mercury surface was constructed usingT. Smith's design. The surfactant (long-chain alkyl-trimethylammonium compounds) in aqueous solution were spread on a clean mercury surface in an atmosphere of helium, and the surface pressure re-areaA and thickness of surface filmd-areaA curves were obtained. The-A curves were characterized by the appearance of multiple inflection points and plateaus, being explained as stepwise dense surface packing of molecules, and the formation of multilayers by film compression with long axes of molecules lying flat on the mercury surface.     

Dendrimer-functionalized self-assembled monolayers as a surface plasmon resonance sensor surface

We report here a multistep route for the immobilization of DNA and proteins on chemically modified gold substrates using fourth-generation NH(2)-terminated poly(amidoamine) dendrimers supported by an underlying amino undecanethiol (AUT) self-assembled monolayer (SAM). Bioactive ultrathin organic films were prepared via layer-by-layer self-assembly methods and characterized by fluorescence microscopy, variable angle spectroscopic ellipsometry, atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and attenuated total internal reflection Fourier transform infrared spectroscopy (ATR-FTIR). The thickness of the AUT SAM base layer on the gold substrates was determined to be 1.3 nm from ellipsometry. Fluorescence microscopy and AFM measurements, in combination with analyses of the XPS/ATR-FTIR spectra, confirmed the presence of the dendrimer/biopolymer molecules on the multilayer sensor surfaces. Model proteins, including streptavidin and rabbit immunoglobulin proteins, were covalently attached to the dendrimer layer using linear cross-linking reagents. Through surface plasmon resonance measurements, we found that sensor surfaces containing a dendrimer layer displayed an increased protein immobilization capacity, compared to AUT SAM sensor surfaces without dendrimer molecules. Other SPR studies also revealed that the dendrimer-based surfaces are useful for the sensitive and specific detection of DNA-DNA interactions. Significantly, the multicomponent films displayed a high level of stability during repeated regeneration and hybridization cycles, and the kinetics of the DNA-DNA hybridization process did not appear to be influenced by surface mass transport limiting effects.     

Surface tension and aggregation properties of novel cationic gemini surfactants with diethylammonium headgroups and a diamido spacer

A series of novel cationic gemini surfactants with diethylammonium headgroups and a diamido spacer were synthesized, and their surface and bulk properties were investigated by surface tension, electrical conductivity, fluorescence, viscosity, dynamic light scattering (DLS), and transmission electron microscopy (TEM) measurements. An interesting phenomenon, that is, the obvious decline in surface tension upon increasing concentration above the critical micelle concentration (cmc), was found in these gemini surfactant solutions, and two explanations were proposed. This surface tension behavior could be explained by the rapid increase in the counterion activity in the bulk phase or the continued filling of the interface with increasing surfactant concentration above the cmc. More interestingly, not only vesicles but also the surfactant-concentration-induced vesicle to larger aggregate (spongelike aggregate) transition and the salt-induced vesicle and spongelike aggregate to micelle transition were found in the aqueous solutions of these gemini surfactants. The spongelike aggregate that is first reported in the cationic gemini surfactant-water binary system is probably caused by the adhesion and fusion of vesicles at high surfactant concentration.     

Fabrication of self-assembled monolayers exhibiting a thiol-terminated surface

We compare two different strategies for fabricating self-assembled monolayers (SAMs) exhibiting a thiol-terminated organic surface, which recently has attracted considerable interest with regard to the binding of metal, and in particular gold particles, to these organic surfaces. Results obtained by IR spectroscopy and scanning tunneling microscopy (STM) reveal that the straightforward approach, namely, using the corresponding organodithiols, typically leads to the formation of disordered, ill-defined surfaces. We demonstrate that high-quality organothiolate adlayers exhibiting an SH-terminated surface can be prepared using dithiols where one of the thiol groups is protected by a thioester group. After formation of the organothiolate adlayer, the protecting group can be removed by immersion into NaOH solution. IR spectra recorded for the deprotected organothiolate adlayer reveal the presence of a highly oriented organothiolate adlayer.     

Thermally controlled preferential molecular aggregation state in a thiacarbocyanine dye

Herein we report the experimental and theoretical study of the temperature dependence of a thiacarbocyanine dye in its monomer, H- and J-aggregates states. We demonstrate the ability to control the ratio of monomer, H- and/or J-aggregates with heat. We link such a control to the conformation dependence of the molecule. An alternative way to gain access to the dominating species without changing the concentration as a complete switching mechanism between all the present species is proposed. The results presented in this work lead to a better understanding of thiacarbocyanine dye's behavior.     

Thermodynamic and aggregation properties of aza- and imino-substituted gemini surfactants designed for gene delivery

Improving the efficiency of gene delivery by using non-viral vectors is currently an area of considerable research interest. Novel derivatives of gemini surfactants having aza- (12-5N-12, 12-7N-12, 12-8N-12) and imino- (12-7NH-12) substituted spacer groups and C12 tails have been designed to improve DNA transfection. Physicochemical characterization of micelle and interfacial properties of these cationic compounds are reported. The effect of these substitutions on the aggregation properties of the gemini surfactants is discussed in the context of results for the 12-s-12 and 12-EOx-12 gemini series, previously reported in the literature. Aza substitution results in a spacer of intermediate hydrophobicity to the above series, reflected by the magnitude of both the critical micelle concentrations and head group areas. Enthalpy and apparent molar volume of micellization data illustrate the differences in the aggregation properties that result from the bulkier and more hydrophobic aza-substituent in the spacer as compared to an ether oxygen (for the 12-EOx-12 series) containing spacer. The 12-7N-12 and 12-8N-12 compounds show aberrant features in the surface tension and enthalpy of dilution results that are not observed for the 12-5N-12 and 12-7NH-12 compounds. Premicelle association is considered to be a source of this behaviour.