Platform for enhanced detection efﬁciency in luminescence-based sensors R. Blue, N. Kent, L. Polerecky, H. McEvoy, D. Gray and B.D. MacCraith Luminescence-based biochip measurement platforms are employed in a wide range of biological applications, such as biomedical diagnos- tics. Based on an understanding of the anisotropic emission properties of luminescence emitters close to a dielectric interface, a simple strategy for producing a better than 25-fold enhancement of the detected luminescence is presented. This strategy is demonstrated for low cost polymer platforms compatible with mass-production. Introduction: There is an increasing requirement to detect a wide range of analytes with high sensitivity in areas such as biomedical diagnostics, environmental monitoring, food quality testing and bioprocess monitoring. Biosensor and chemical sensor technologies play a signiﬁcant role in these ﬁelds [1]. Of the variety of sensing schemes developed, optical techniques are widespread and, within these, luminescence-based systems are the most widely used owing primarily to their better sensitivity than alternative approaches. Fluorescent labels (such as cyanine dyes and quantum dots) are employed routinely in bio-assays to transduce the biomolecular binding event. Array biosensors, which typically comprise patterned arrays of biorecognition elements (such as antibodies) on a planar dielectric surface, provide multi-analyte and=or multi-replicate measurement capability. Such sensors, often referred to as microarray biochips, are of signiﬁcant interest, especially in applications such as point-of-care testing, water quality monitoring and biowarfare detection [2]. Applications such as these are driving the demand for low cost, portable instruments combined with microarray biochips. The biochips are typically fabricated from glass or polymer, often incorporate microﬂuidic functionality, and are treated as single-use disposable components. In this Letter we report for the ﬁrst time a novel platform which provides a dramatic enhancement of the detected luminescence signal from an emitting species located on or close to a dielectric substrate. This platform is generic in nature and is compatible with microfabri- cated, mass produced biochips, especially in the case of polymer materials. The enhancement strategy presented here has major signiﬁ- cance for the future design and fabrication of low-cost, high sensitivity platforms. In particular, it will lead to lower limits of detection and enable the use of lower cost detection systems, such as CMOS in preference to CCD cameras. In this work we will refer in general to luminescence although most biosensors employ emitting species with short lifetimes, in which case the more speciﬁc term ﬂuorescence applies. Theoretical background: Experimental evidence of the anisotropic emission properties of electric dipoles close to a dielectric interface has been reported experimentally by a number of authors [3, 4], together with a qualitative theory of the observations. More recently, a more rigorous electromagnetic theory has been proposed [5] to explain the phenomenon. We have subsequently expanded upon this work and developed a theory [6] that predicts the anisotropic emission pattern from systems employing thin luminescent ﬁlms on planar dielectric substrates, which are used widely in the area of optical chemical sensors and biosensors. The theory models the spatial distribution of the luminescence radiated by randomly distributed luminescent molecules across multilayer systems comprising arbitrary linear, isotropic media. For example, the particular case where the thin luminescent layer is sol-gel-derived silica doped with a luminescent indicator and deposited on a glass substrate is shown in Fig. 1. This Figure illustrates the spatial emission pattern as a 2-D cross-section. A comparison of Fig. 1a and b indicates the sensitivity of the mission pattern to the superstrate refractive index. In both cases, however, it is clear that a substantial portion of the luminescence is radiated into the glass substrate, which has a higher refractive index than the environment above it. Of most signiﬁcance is the fact that the majority of the light that is radiated into the substrate is so-called supercritical angle luminescence, or more speciﬁcally supercritical angle ﬂuorescence (SAF) [7], i.e. it propagates at angles above the critical angle corresponding to the environment (e) substrate (s) interface y es c ¼ arcisnðn e =n s Þ ð1Þ air a b air glass substrate q ls c q as c luminescent spot air water glass substrate luminescent spot q ls c q ls c q as c q ls c q as c q ws c q ws c q as c Fig. 1 Angular properties of luminescence radiated from a thin luminescent spot deposited on a glass substrate The substrate is surrounded by air below and by air (a) or water (b) above. The solid lines represent the angular distributions of luminescence radiated from luminescent spots of thickness t 1 ¼ 0.1 l, and the dotted lines the location of the relevant critical angles, where 1 ¼ layer, a ¼ air, and w ¼ water Consequently, in the case of planar substrates with parallel sides as shown, this light will undergo total internal reﬂection at the bottom interface and become substrate-conﬁned (SC) or trapped. In a ‘conven- tional’ detection technique, where a detector is placed above or below the substrate in the vicinity of the luminescent layer, it is clear that a signiﬁcant amount (the majority) of the generated luminescent signal will be waveguided away as SC modes and will go undetected. detector air air a SC modes Fig. 2 Cross-section of the conﬁguration facilitating improved efﬁciency of the luminescence capture The SC modes radiated from the luminescent spot are redirected towards the detector placed below the substrate by means of a total internal reﬂection at the interface A Enhanced luminescence capture system: The phenomenon described above is very signiﬁcant when one considers the widespread employ- ment of planar substrates coated with one or more ﬂuorescence-based sensors. It is clear that conventional systems, which do not take account of SAF, are highly inefﬁcient as a result. To address this issue, we have developed a range of sensor conﬁgurations [8] to produce a much greater capture efﬁciency. These conﬁgurations can be realised easily in a wide range of polymers using standard micro-fabrication techniques (such as micro-injection moulding). In each case, our theoretical model, combined with knowledge of the relevant refractive indices (polymer substrate, superstrate), enables us to calculate the spatial emission pattern of the ﬂuorescent molecules. Here we describe one such conﬁguration that has been implemented. The concept is shown in Fig. 2. Injection moulding is used to produce chips with an array of frustrated cones such as shown in Fig. 2. The cone angle a is chosen so that SC modes radiated from the luminescent spot undergo total internal reﬂection at the side-walls of the cone. The advantage of this conﬁguration is two-fold. First, the total internal reﬂection applies to all the SC modes and, second, the redirected SC modes impinge on the bottom interface of the substrate at angles close to 0  . Conse- quently, a large fraction of the power is transmitted out of the substrate towards the detector. ELECTRONICS LETTERS 9th June 2005 Vol. 41 No. 12