On-chip light sheet illumination for nanoparticle tracking in microfluidic channels

A simple and inexpensive method is presented to efficiently integrate light sheet illumination in a microfluidic chip for dark-field microscopic tracking and sizing of nanoparticles. The basic idea is to insert an optical fiber inside a polydimethylsiloxane (PDMS) elastomer microfluidic chip and use it as a cylindrical lens. The optical fiber is in this case no longer seen as only an optical waveguide but as a ready-made micro-optical component that is inexpensive and easy to source. Upon insertion, the optical fiber stretches the PDMS microchannel walls, which has two effects. The first effect is to tone down the intrinsic ripples in the PDMS that would otherwise create inhomogeneities in the light sheet illumination. The second effect is to remove any obliqueness of the channel wall and constrain it to be strictly perpendicular to the propagation of the illumination, avoiding the formation of a prismatic diopter. Through calculations, numerical simulations and measurements, we show that the optimal configuration consists in creating a slowly converging light sheet so that its axial thickness is almost uniform along the tracked area. The corresponding thickness was estimated at 12 μm, or 10 times the depth of field of the optical system. This leads to an at least six-fold increase in the signal-to-noise ratio compared to the case without the cylindrical lens. This original light-sheet configuration is used to track and size spherical gold nanoparticles with diameters of 80 nm and 50 nm.

Determination of photoluminescence quantum yields in dilute solution using non-monochromatic excitation light

The photoluminescence (PL) quantum yields (QYs) of fluorophores in dilute solutions can be determined fluorimetrically according to the comparative method employing standards of known PLQY. This method has recently been demonstrated to become more robust when the absorption of the excitation light and the PL emission are measured simultaneously using a transmitted light detector integrated in the fluorimeter. Herein, aided by fiber-coupled spectroscopic equipment and computerized data processing, we elaborate on this method by measuring the full corrected intensity spectrum of the excitation light transmitted through the sample. This further releases constraints on the monochromatic character of the excitation light and enables the use of broad-band excitation sources such as light-emitting diodes (LEDs). Furthermore, the protocol includes measurements at increasing dye concentration, rigorously verifying the required proportionality between absorbed and emitted light intensities. The PLQYs of solutions of fluorophores determined using the new method are in close agreement with published values.

Spectroscopic and Hydrodynamic Characterisation of DNA-Linked Gold Nanoparticle Dimers in Solution using Two-Photon Photoluminescence

Two-photon photoluminescence (TPPL) emission spectra of DNA-gold nanoparticle (AuNP) monoconjugates and the corresponding DNA-linked AuNP dimers are obtained by photon time-of-flight spectroscopy. This technique is combined with two-photon photoluminescence fluctuation correlation spectroscopy (TPPL-FCS) to simultaneously monitor the optical and hydrodynamic behaviour of these nano-assemblies in solution, with single-particle sensitivity and microsecond temporal resolution. In this study, the AuNPs have an average core diameter of 12 nm, which renders their dark-field plasmonic light scattering too weak for single-particle imaging. Moreover, as a result of the lack of plasmonic coupling in the dimers, the optical extinction, scattering and photoluminescence spectra of the DNA-AuNP complexes are not sufficiently different to distinguish between monomers and dimers. The use of TPPL-FCS successfully addresses these bottlenecks and enables the distinction between AuNP monomers and AuNP dimers in solution by measurement of their hydrodynamic rotational and translational diffusion.

Optical extinction and scattering cross sections of plasmonic nanoparticle dimers in aqueous suspension

Absolute extinction and scattering cross sections for gold nanoparticle dimers were determined experimentally using a chemometric approach involving singular-value decomposition of the extinction and scattering spectra of slowly aggregating gold nanospheres in aqueous suspension. Quantitative spectroscopic data on plasmonic nanoparticle assemblies in liquid suspension are rare, in particular for particles larger than 40 nm, and in this work we demonstrate how such data can be obtained directly from the aggregating suspension. Our method can analyse, non invasively, the evolution of several sub-populations of nanoparticle assemblies. It may be applied to other self-assembling nanoparticle systems with an evolving optical response. The colloidal systems studied here are based on 20, 50 and 80 nm gold nanospheres in aqueous solutions containing sodium lipoate. In these systems, the reversible dimerisation process can be controlled using pH and ionic strength, and this control is rationalised in terms of DLVO theory. The dimers were identified in suspension by their translational and rotational diffusion through scattering correlation spectroscopy. Moreover, their gigadalton molecular weight was measured using electrospray charge-detection mass spectrometry, demonstrating that mass spectrometry can be used to study nanoparticles assemblies of very high molecular mass. The extinction and scattering cross sections calculated in the discrete-dipole approximation (DDA) agree very well with those obtained experimentally using our approach.

Photoluminescence spectra and quantum yields of gold nanosphere monomers and dimers in aqueous suspension

Photoluminescence spectra and quantum yields of gold spherical monomers and dimers mixed in solution.

The intrinsic luminescence of individual plasmonic nanostructures in aqueous suspension by photon time-of-flight spectroscopy

We have studied the intrinsic one-photon excited luminescence of freely diffusing gold nanoparticles of different shapes in aqueous suspension. Gold nanospheres were used as a reference, since their luminescence has been investigated previously and their light absorption and scattering properties are described analytically by Mie theory. We then studied gold nanobipyramids and nanostars that have recently gained interest as building blocks for new plasmonic nanosensors. The aim of our study is to determine whether the luminescence of gold nanoparticles of complex shape (bipyramids and nanostars) is a plasmon-assisted process, in line with the conclusions of recent spectroscopic studies on spheres and nanorods. Our study has been performed on particles in suspension in order to avoid any artefact from the heterogeneous environment created when particles are deposited on a substrate. We employ a recently developed photon time-of-flight method in combination with correlation spectroscopy of the light scattered by the particles to probe the luminescent properties of individual particles based on a particle-by-particle spectral analysis. Furthermore, we have performed resonant light scattering spectroscopic measurements on the same samples. Our work demonstrates the power of our time-of flight method for uncovering the plasmonic signatures of individual bipyramids and nanostars during their brief passage in the focal volume of a confocal set-up. These spectral features of individual particles remain hidden in macroscopic measurements. We find that the intrinsic photoluminescence emission of gold bipyramids and gold nanostars is mediated by their localized surface plasmons.

Resonant light scattering spectroscopy of gold, silver and gold-silver alloy nanoparticles and optical detection in microfluidic channels

Dark field resonant light scattering by gold and silver nanoparticles enables the detection and spectroscopy of such particles with high sensitivity, down to the single-particle level, and can be used to implement miniaturised optical detection schemes for chemical and biological analysis. Here, we present a straightforward optical spectroscopic methodology for the quantitative spectrometric study of resonant light scattering (RLS) by nanoparticles. RLS spectroscopy is complementary to UV-visible absorbance measurements, and we apply it to the characterisation and comparison of different types of gold, silver and gold-silver alloy nanoparticles. The potential of gold and silver particles as alternatives for fluorescent probes in certain applications is discussed. RLS spectroscopy is shown to be useful for studying analyte-induced gold nanoparticle assembly and nanoparticle chemistry, which can induce radical changes in the plasmonic resonances responsible for the strong light scattering. Furthermore, the feasibility of dark field RLS detection and quantitation of metal nanoparticles in microfluidic volumes is demonstrated, opening interesting possibilities for the further development of microfluidic detection schemes.

Foam films in the presence of functionalized gold nanoparticles

Dry aqueous foams made of anionic surfactant (SDS) and spherical gold nanoparticles are studied by small angle X-ray scattering and by optical techniques. To obtain stable foams, the surfactant concentration is well above the critical micelle concentration. The specular reflectivity signal obtained on a very thin film (thickness 20 nm) shows that functionalized nanoparticles (17 nm typical size) are trapped within the film in the form of a single monolayer. In order to isolate the film behavior, investigations are made on a single film confined in a tube. The film thinning according to the ratio of functionalized nanoparticle and SDS micelles (1:1, 1:10, 1:100) is mainly governed by the structural arrangement of SDS micelles. In thick films, nanoparticles tend to form aggregates that disappear during drainage. In particular self-organization of nanoparticles (with different surface charge) inside the film is not detected.

Quantitative full-colour transmitted light microscopy and dyes for concentration mapping and measurement of diffusion coefficients in microfluidic architectures

A simple and versatile methodology has been developed for the simultaneous measurement of multiple concentration profiles of colourants in transparent microfluidic systems, using a conventional transmitted light microscope, a digital colour (RGB) camera and numerical image processing combined with multicomponent analysis. Rigorous application of the Beer-Lambert law would require monochromatic probe conditions, but in spite of the broad spectral bandwidths of the three colour channels of the camera, a linear relation between the measured optical density and dye concentration is established under certain conditions. An optimised collection of dye solutions for the quantitative optical microscopic characterisation of microfluidic devices is proposed. Using the methodology for optical concentration measurement we then implement and validate a simplified and robust method for the microfluidic measurement of diffusion coefficients using an H-filter architecture. It consists of measuring the ratio of the concentrations of the two output channels of the H-filter. It enables facile determination of the diffusion coefficient, even for non-fluorescent molecules and nanoparticles, and is compatible with non-optical detection of the analyte.

Fluorescence correlation spectroscopy reveals strong fluorescence quenching of FITC adducts on PEGylated gold nanoparticles in water and the presence of fluorescent aggregates of desorbed thiolate ligands

Colloidal gold particles functionalised with oligoethylene-glycolated disulfide ligands and fluorescent moieties derived from fluorescein isothiocyanate (FITC) have been prepared and studied in aqueous suspension using fluorescence correlation spectroscopy (FCS). FCS probes the dynamics of the particles at the single object level, and reveals the desorption of fluorescent ligands which subsequently aggregate into larger (slower diffusing) objects. Cross-correlation spectroscopy of the FITC fluorescence and the Rayleigh-Mie scattering (RM-FCCS) of the gold cores shows that the only detectable fluorescent objects are free ligands and aggregates not associated with a gold particle. The fluorescence of bound fluorophores is quenched making their fluorescence too weak to be detected. FCS and RM-FCCS are useful tools for characterising functionalised noble metal particles in solution, under conditions similar to those used in optical bio-imaging. Desorption of thiolates from gold nanoparticles needs to be taken into account when working with these materials at low concentration.

Probing the interactions between disulfide-based ligands and gold nanoparticles using a functionalised fluorescent perylene-monoimide dye

The binding of disulfides to gold nanoparticles was investigated using fluorescence spectroscopy and a perylene-monoimide dye coupled to a dissymmetric disulfide via a tetraethyleneglycolalkyl chain (PMImSS). Quantum chemical calculations using the polarizable continuum model (PCM) predict a strong quenching of perylene-monoimide fluorescence by gold nanoparticles as a result of efficient excitation energy transfer from the dye to the particle. Such quenching is indeed observed when unfunctionalised gold nanoparticles are added to a solution of PMImSS. The fluorimetric titration curves show behaviour indicative of the existence of an equilibrium between free and bound ligands (association constant 5 x 10(5) M(-1)), whereas the affinity of thiols and disulfide for gold surfaces is in general assumed to be much higher. Gold nanoparticles fully functionalised with PMImSS were synthesised and purified. Fluorescence correlation spectroscopy shows the appearance of free PMImSS ligands in dilute (approx. pM) suspensions of these PMImSS-functionalised nanoparticles over a period of several days.

Two-photon transitions in quadrupolar and branched chromophores: experiment and theory

A combined experimental and theoretical study is conducted on a series of model compounds in order to assess the combined role of branching and charge symmetry on absorption, photoluminescence, and two-photon absorption (TPA) properties. The main issue of this study is to examine how branching of quadrupolar chomophores can lead to different consequences as compared to branching of dipolar chromophores. Hence, three structurally related pi-conjugated quadrupolar chromophores symmetrically substituted with donor end groups and one branched structure built from the assembly of three quadrupolar branches via a common donor moiety are used as model compounds. Their photophysical properties are studied using UV-vis spectroscopy, and the TPA spectra are determined through two-photon excited fluorescence experiments using femtosecond pulses in the 500-1000 nm range. Experimental studies are complemented by theoretical calculations. The applied theoretical methodology is based on time-dependent density functional theory, the Frenkel exciton model, and analysis in terms of the natural transition orbitals of relevant electronic states. Theory reveals that a symmetrical intramolecular charge transfer from the terminal donating groups to the middle of the molecule takes place in all quadrupolar chromophores upon photoexcitation. In contrast, branching via a central electron-donating triphenylamine moiety breaks the quadrupolar symmetry of the branches. Consequently, all Frank-Condon excited states have significant asymmetric multidimensional charge-transfer character upon excitation. Subsequent vibrational relaxation of the branched chromophore in the excited state leads to a localization of the excitation and fluorescence stemming from a single branch. As opposed to what was earlier observed when dipolar chromophores are branched via the same common electron-donating moiety, we find only a slight enhancement of the maximum TPA response of the branched compound with respect to an additive contribution of its quadrupolar branches. In contrast, substantial modifications of the spectral shape are observed. This is attributed to the subtle interplay of interbranch electronic coupling and asymmetry caused by branching.

Quenching of molecular fluorescence on the surface of monolayer-protected gold nanoparticles investigated using place exchange equilibria

The insertion of fluorescently labeled thiols into the protecting self-assembled monolayer on the surface of gold nanoparticles through place exchange reactions and the effects of this insertion on the photophysical properties of the fluorophores are investigated. Analysis of solution-phase fluorescence data using a dynamic equilibrium model yields the equilibrium constant for the place exchange equilibrium, as well as the relative fluorescence brightness of the fluorophores on the particle surface. In all cases we find a significant quenching of the fluorescence, and potential reasons for this quenching are discussed. In the case of these relatively small particles (4.5 nm diameter), the quenching appears to be mainly related to enhanced nonradiative deactivation pathways. The place exchange equilibrium constant reveals a reduced affinity of the fluorescently labeled thiols for insertion into the nonfluorescent alkylthiol monolayer (K(eq) approximately 0.2) compared to unlabeled alkylthiols.

A NADPH substitute for selective photo-initiation of reductive bioprocesses via two-photon induced electron transfer

A NADPH substitute where the nicotinamide moiety is replaced by a chromophoric unit having much larger two-photon absorption cross-section and able to transfer electrons to flavins only upon excitation is described as an effective two-photon nanotrigger for selective photo-activation of electron transfer in bioreductive processes.

Distance-dependent fluorescence quenching on gold nanoparticles ensheathed with layer-by-layer assembled polyelectrolytes

We report on the preparation, characterization, and photophysical study of new fluorescent core/shell nanoparticles fabricated by electrostatic layer-by-layer assembly. On the basis of gold cores with a diameter of 13 nm, these nanocolloids possess different fluorescently labeled polymer corona layers at various distances from the surface of the core metal using nonfluorescent polyelectrolytes as spacer layers. UV-visible spectroscopy and transmission electron microscopy confirm that the particle suspensions of fluorescently labeled core/shell nanoparticles are stable at all stages of their construction. Photophysical investigations reveal strongly distance-dependent fluorescence quenching in these particle systems. The contribution of the metal core to this quenching can be assesed precisely after the gentle dissolution of the gold cores by potassium cyanide. The photophysical measurements reveal clearly that the gold nanoparticles decrease the transition probability for radiative transitions.

A modular approach to two-photon absorbing organic nanodots: brilliant dendrimers as an alternative to semiconductor quantum dots?

Nanoscopic fluorescent dendrimers having up to 96 two-photon chromophores and showing very large two-photon absorption cross-sections (up to 56 000 GM) were designed as a complementary "organic" alternative to quantum dots.

Effects of (Multi)branching of Dipolar Chromophores on Photophysical Properties and Two-Photon Absorption

To investigate the effect of branching on linear and nonlinear optical properties, a specific series of chromophores, epitome of (multi)branched dipoles, has been thoroughly explored by a combined theoretical and experimental approach. Excited-state structure calculations based on quantum-chemical techniques (time-dependent density functional theory) as well as a Frenkel exciton model nicely complement experimental photoluminescence and one- and two-photon absorption findings and contribute to their interpretation. This allowed us to get a deep insight into the nature of fundamental excited-state dynamics and the nonlinear optical (NLO) response involved. Both experiment and theory reveal that a multidimensional intramolecular charge transfer takes place from the donating moiety to the periphery of the branched molecules upon excitation, while fluorescence stems from an excited state localized on one of the dipolar branches. Branching is also observed to lead to cooperative enhancement of two-photon absorption (TPA) while maintaining high fluorescence quantum yield, thanks to localization of the emitting state. The comparison between results obtained in the Frenkel exciton scheme and ab initio results suggests the coherent coupling between branches as one of the possible mechanisms for the observed enhancement. New strategies for the rational design of NLO molecular assemblies are thus inferred on the basis of the acquired insights.

Strong modulation of two-photon excited fluorescence of quadripolar dyes by (de)protonation

Two quadripolar dyes have been designed and synthesized that present large cross sections for two-photon excitation and whose fluorescence responds strongly to (de)protonation. These dyes are considered as prototypes of molecular pH probes for multiphoton fluorescence microscopy.

Making sense of lanthanide luminescence

The luminescence of trivalent lanthanide ions has found applications in lighting, lasers, optical telecommunications, medical diagnostics, and various other fields. This introductory review presents the basics of organic and inorganic luminescent materials containing lanthanide ions, their applications, and some recent developments. After a brief history of the discovery, purification and early spectroscopic studies of the lanthanides, the radiative and nonradiative transitions of the 4f electrons in lanthanide ions are discussed. Lanthanide-doped phosphors, glasses and crystals as well as luminescent lanthanide complexes with organic ligands receive attention with respect to their preparation and their applications. Finally, two recent developments in the field of luminescent materials are addressed: near-infrared luminescent lanthanide complexes and lanthanide-doped nano-particles.

Action cross sections of two-photon excited luminescence of some Eu(iii) and Tb(iii) complexes

Four different luminescent lanthanide complexes have been studied with respect to multiphoton excitation using near-infrared femtosecond pulses. The method for measuring action cross sections of two-photon excited fluorescence in solution relative to a known standard is reviewed. Two refractive index-related corrections are necessary in this method: one for the multiphoton excitation process, the other for the collection of the emitted light. It has been found that (2,4,6-trimethoxyphenyl)dipicolinic acid and Michler's ketone are reasonable sensitisers of two-photon excited lanthanide luminescence in solution, whereas dipicolinic acid and carbostyril-124 do not give rise to any detectable two-photon excited lanthanide luminescence using modest excitation powers (<20 mW focused at the sample) in the 700-1000 nm range.

Towards “smart” multiphoton fluorophores: strongly solvatochromic probes for two-photon sensing of micropolarity

New fluorophores, combining broad, very high two-photon absorption in the near-infrared region with a marked dependence of their emission spectra on solvent polarity, have been designed as model probes for two-photon sensing of the chemical environment.

Versatility of aqueous micellar solutions for self-assembled monolayers engineering

Self-assembly of aliphatic as well as aromatic thiol-terminated molecules was achieved onto a variety of gold surfaces using aqueous micellar solutions. Scanning tunneling microscopy experiments allowed us to demonstrate that the increase in the density of self-assembled monolayers (SAMs) prepared from micellar aqueous solvent compared to that prepared from ethanol directly originates from the decrease in defect density in the SAM (etch pits, domain boundaries) and not from a denser local packing of the molecules. Extending the use of such an aqueous solvent to various conjugated molecules, we report for the first time the insertion of these molecules from an aqueous solution in a dodecanethiol (DT) SAM and the ligand-exchange on the surface of DT stabilized gold nanoparticles deposited as a Langmuir-Blodgett film. Finally, we show that aqueous micellar DT solutions allow the preparation of DT SAMs on gold through a micropatterned resist mask. These results make possible the use of water to deliver molecules on a solid substrate to build molecular devices in a way compatible with lithography requirements in microelectronic processes.

Organization and orientation of amphiphilic push-pull chromophores deposited in Langmuir-Blodgett monolayers studied by second harmonic generation and atomic force microscopy

Orientation and organization of two amphiphilic push-pull chromophores mixed with two phospholipids (dipalmitoylphosphatidylcholine and dioleoylphosphatidylcholine) in Langmuir-Blodgett (LB) monolayers are investigated by second harmonic generation. The LB monolayers have also been characterized by atomic force microscopy and UV-vis spectroscopy. The effective molecular orientations and hyperpolarizabilities of the chromophores are studied as a function of the phospholipid concentrations. The experimental results are discussed within the frame of a model of orientational distribution of the chromophores which gives the orientational mean angle and bounds on the orientational disorder. The mean orientation of the chromophores is found to be within 45-55 degrees whereas their hyperpolarizability coefficients, measured with respect to quartz, are estimated to be in the range (0.3-0.7) x 10(-27) esu taking account of the maximal orientational disorder.

Excimer probe of the binding of alkyl disulfides to gold nanoparticles and subsequent monolayer dynamicsElectronic supplementary information (ESI) available: Absorption spectra of nanoparticle solutions in toluene. See http://www.rsc.org/suppdata/pp/b3/b310952f/

A symmetric alkyl disulfide bearing two pyrenyl chromophores allows for fluorescence monitoring of its binding to gold nanoparticles and the subsequent diffusion of the geminate thiolates on the particle surface.